XnView is a photo editing program which supports a large number of graphic formats, probably more than 400. This is a perfect program to view, manage and light editing of pictures. XnView is useful for converting into different formats like GIF, BMP, JPG, PNG and Tiff.
If you are looking for a free photo editing which is not too complicated, Photoscape is an easy to use editing software. You can view and edit images and make animated gif. The special Photoscape interface makes it an outstanding photo editor and viewer.
GIMP is a photo editing program which is free and very comprehensive. GIMP means GNU Image Manipulation Program and was originally developed for Linux software, but for years also available for Windows and Mac. OSX can compete with Photoshop, the well-known and very expensive software.