Not exactly a Photoshop killer, but it does have some of the same features and is as free as they come with it's open source license.
Supports ranges from the common likes of JPEG (JFIF), GIF, PNG, TIFF to special use formats such as the multi-resolution and multi-color-depth Windows icon files
And... Thanks to the transparent virtual file system, it is possible to load and save files to from remote locations using protocols such as FTP, HTTP or even SMB (MS Windows shares) and SFTP/SSH.
It also runs on all of the platforms listed below:
- GNU/Linux (i386, PPC)
- Microsoft Windows (XP, Vista)
- Mac OS X
- Sun OpenSolaris