Hello Jonathon Dearborn!
I'd like to start by saying that I can see some real potential with the application so far, there are a few things that I would personally change or allow options to alter and some possible bugs or oversights, but I will get to that shortly. So far it's quite a nifty little development and I just happened to stumble upon it randomly, thought I'd give it a test run and provide some feedback.
A little overview of my experience so far...
Generally speaking I couldn't use this in any real use case scenario as it is, but with a little work and a few updates I could see this being something that anyone could use efficiently for creating static sprites, tiles or quick concepts without all of the clutter that some other applications provide. With that being said, the main reason is solely due to efficiency. I've found myself clicking more and more (even with the cool little quick access menu) for things that I shouldn't have to click so often to achieve. I did have a few issues regarding the colour palette and I will explain in more detail below but all in all, I enjoy the concept behind the tool, and I would use it again after some updates!
Some of the icons where really hard to make out what they did, hovering over them made it clear for most but I still found myself struggling to grasp what I needed on the fly when I was quickly throwing up the menu and looking for a tool "quickly" but aside from that the animated icons I think are a little much, perhaps a small 1 pixel offset in scaling from the darker to lighter colours that are currently used might be a more subtle and defining "this is selected" animation. Just a minor suggestion. The icons are something that I would focus on, make them a little more definable or easy to read as a whole.
If you could improve the quick access menu to be naturally quicker to grab tools on the fly, I would propose something along the lines of a pie-menu that can expand for additional choices of tools, this would be quicker and allow to quickly select the previously selected tool type and potentially avoid the issues of mixing up which icon is for what.
Animated Background Image
This is really off putting while working on a sprite, I liked that I could zoom out after through the process and see that though, is there any ability to add a custom image, or the ability to turn it off, I would prefer to be able to toggle it on or off more than use a custom image.
This is where I noticed the most issues when using Pixel Proof. First of all, I love the scrolling through colours using the "[" and "]" keys. Although it doesn't seem to be based on the actual palette created by the user. When I added colours to the quick menu (the only place to add colours) I wasn't able to scroll through them with those hotkeys. Also, when I click the "+" to add a new colour to the palette, the window to select the colour to add pops up in the top left corner of the screen, not the window of the application or the center of my screen which would be preferred if I had to chose, it would be best if you could set this to remember the last location for more convenience. Mostly so far these issues I have been pointing out have been fairly minor and what I would consider to be easily fixed (within reason) but one thing that got me worse than all of these minor annoyances was the fact that when I was experimenting with the software I entered "viewer mode" and couldn't get back out of it. The notifications or help for such things isn't really around from what I saw, but I hit "ESC" as I figured that would cancel the mode but it closed the entire application. This wasn't much of an issue as a whole, but when I loaded the file back up I was really annoyed at the fact that the palette that was frustrating to create was not tied to the save file that I created, so I had the default colours and no real way to easily recreate the palette that I use, so instead I ended up using the default colours and couldn't be bothered enough to finish my sprite concept in Pixel Proof.
Selection & Moving
For the life of me, I couldn't figure out how to move the selection set I had made, which was really annoying. I did try a variety of things but if this doesn't exist, you should definitely work towards adding the ability to move a selection. I am not refering to the "Truck" tool either, that is a really nice feature but not what I am talking about right here.
I really do love some of the features, the pixel perfect "pixel tool" is nice, makes drawing things easier requiring less editing after when regarding curved low resolution pencil strikes. Also, the ability to select which tools are tied to the mouse buttons is a very nice feature, some applications allow the right mouse click to grab a colour others force it to erase and there is no easy way to alter, this software allows you to change to any tool as required per task, I love that! -- The ability to pan the tile or sprite to create a better tiling sprite is something that many pixel artists have grown to love about other software and this is definitely a great feature to have, it will help a lot.
A layer system would be nice for testing ideas on sprite concepts out and save some time when trying shading details over tiles and so on without committing to them right off the bat.
I do have more to say about this little software and more suggestions that could help improve it, but I don't have as much time to write it all up right now. These are some of the simpler things to address and issues that many will likely face so I thought that I'd stop by and leave you some feedback to take on and work with.
I am fond of this, I would love to see it grow and I look forward to some kind of updates. Even with the few shortcomings and annoyances, it's more pleasant to work with than some things out there, once a few of the above mentioned issues are fixed it could be a really speedy way to create a sprite concept on the fly without an over bearing interface which would help when trying to teach someone pixel art too!
Jonathon Dearborn, I may be in touch via email some time soon. However I am fairly busy as of late so there may be some delay in me writing. I hope this feedback helps you in any way and I'm open to chatting to further assist with testing if required.
Please keep up the great work. Oh and I nearly forgot, here is the sprite using mostly the default colours before I decided to give up on it: (nothing special, but it allowed me to test out the features of the software and provide feedback)