I wanted to present you my picture viewer: Tom's Editor Desktop.
It supports almost 250 formats, including a whole lot of formats from Atari. Every loaded image can be saved as BMP, PNG, PNM, GIF or JPEG.
The program is freeware. Really freeware which means you can use it commercially and there are no better, paid version.
Right now it works only under Windows, although under OSX and Linux you should be able to run it with Wine.
If you have some graphic my program can't handle, you can send them to me and I will try my best to write a decoder for them.
Direct link to the EXE file (500 kB). It does't require installation. Works under Windows 32/64 bit:http://tomseditor.com/blog/TomsEditorD.exe