I tried loading some images as individual frames, and yes that worked immediately.
But trying to load as "sprite sheet" gives me the sprite sheet import window, but the area where the sprite sheet should be is blank, and the image dimensions listed as "0x0" in the corner.
And similar with loading GIF, except the "before" and "after" windows are just blank.
Seeing as the Individual Frames mode works fine, my guess is that this is some sort of bug.
EDIT: I found out exactly what's causing the problem.
I had a hunch it might have to do with different Drive Letters, and it does. I like to place my portable apps on an external thumb drive located at "Z:\TOOLS\" so this program would be located in "Z:\TOOLS\BitmapFlow\BitmapFlow.exe"
If the program is on the "Z:" drive, it won't load any images from the C:" drive, or any other drive. But if try loading a GIF or Sprite Sheet from the "Z:" drive, it DOES import correctly!
To confirm that it was indeed an issue with the drive letter, I copied over BitmapFlow to the "C:" drive -- to "C:\BitmapFlow\" precisely.
Once it was in the "C:" drive, the exact opposite happened, as I predicted -- only GIFs and spritesheets from the "C:" drive would load, and it would no longer load images from the "Z:" drive.
So there's a bug in your code when reading files where it assumes that the files loaded are on the SAME drive letter as the program itself. So if I have your program located at "Z:\TOOLS\BitmapFlow\BitmapFlow.exe" and try to load a file located at "C:\test\sample.gif" -- my guess is that it tries to load "Z:\test\sample.gif" instead -- which this file path doesn't exist, so it fails to load anything (thus a 0x0 image dimension).
Oddly, this bug doesn't seem to happen when loading as "individual frames". So you may have either used a different code there, or fixed it there and not ported the fix to the other methods.
You should be able to test all of this if you try running the program from a USB drive -- or try loading GIFs from a USB drive that isn't in the same letter as the program.