Is there a program for OS X and Linux that I can use to add Textual information to PNG image files? Something for Windows also, maybe?
The "comment" field of PNG files can be edited with GIMP, however I'm not sure about the other fields. To do this in GIMP, open the image and select "Image" from the toolbar, then "Image Properties". From the "Image Properties" dialog, select the "Comment" tab.
GIMP is also available for both Linux, Windows and Mac OS X.