Thanks for your reply!
Unfortunately, I don't know much about color profiles and OpenGL either, so I can't help you with this :(

An option to change color of the black border and another option to set default background color of transparent images will be great! But for fullscreen mode, if you make one, it would be better to leave border color black IMO.

As for subfolders, I also think that it will be useful. Perhaps there could be an option "Look into subfolders", so that when you open a folder, the back/forward buttons will go through the files in all the subfolders. And the status bar could have the name of the current folder. Or another idea: maybe add a side-window with the folder tree, so that you can open it and quickly change the folder, without going into file-open dialog.