Hi Chillax I try to adress all this topics:
1- Working on multi size upload mode
4-Snap view is possible to snap all sides (back, front, left, right top and bottom sides) and all the isometrics .. There is button called Lock view 
that allow the user to not lock the views incase need to work on specific angles.
5- When I do implement different png sizes that popup telling the user that the png is not correct will not appear anymore. :)
9. "Can you make line mode always render the pixels it will draw? Since I noticed atm it only renders them if you move your mouse around.">>>> Sorry I do not understand this point. If u can explain further will help me out :)
13. "A bind for using a key and then using mouse wheel to increase and decrease brush size would be nice." There is a keyboard shortcut atm (numpad 1-0) to change pencil-eraser size... I might include a custom key to adjust that with the mouse wheel.
15- I notice that big sizes over 128 the app is slow and laggy... I tried several times to improve that but not much advance... I will dedicate a full day on it to improve and optimice the app to be able to create 512px size models.
The rest of the topics are on my list to do, hope I answer all your suggestions... Many thanks :)
Nice! Best of luck btw!
I tested Lock View, and it does no longer snap to inbetween angles like SW, NW, W, E, etc, but it does still snap to the top and bottom? which imo it shouldn't snap to any angle at all when disabled since one would assume it gives full free control when disabled but it doesn't.
Also I think should be named "Snap View" or "Snap to Isometric"? Using the word lock sounds like it restricts your view when in this context its more like a helper, not a "camera lock". Also maybe add a tooltip for the right side buttons in general? It would help to make them a bit more clear.
About point 9, oki, so... Basically if you have your mouse on the model, click somewhere to draw point A, then move your mouse to another spot, point B, KEEP THE MOUSE STILL, and hold shift, it should show you the LINE it would make from point A to point B when you pressed shift.Currently, line mode only shows you the line that would be drawn from point A to point B AFTER you move the mouse after you hold the line button, but if the mouse is completely still, it doesn't show you that line connection.
About 13, ohhh, I was unaware, but yeah, I am used to just be able to just "Ctrl + Mouse Wheel" to change pixel size, and thanks! it will be helpful. In general, the idea is to make it so the user doesn't have to mouse their mouse off where they are painting, since it can get a bit tedious to go back and forward over and over.
OH, in that note, could you make their tooltips also tell you their bind perhaps? That might also be an easy way to make things a bit nicer too, since you just hover over and see the bind it has.
Also I know this is just me being me, but if you do ever need an extra hand developing the tool, in the form of PRs, suggestions of how to approach certain things, optimization tips (which I think you can tell I worry a lot about because it is also one of my favorite things to do when programming lol), or if you need help in general, I am willing to help as I myself develop apps, modding tools, games and tools for game development. If not, it's okey, but I figured to throw that out there. c: