Use a graphic editor and save the image as a .gif . Let the theme builder software convert the image back to .png during the process of saving the theme.
That will most likely fix that error message, in the event it does not then depending on your graphic editor try saving the image for web pages and again let the editor reduce the file size and again save it as a .gif file.
That should do the trick.

There are many posts on the topic but off the tip of my fingers i don't have any links to them.

try this thread:
http://www.blackberryforums.com/ring...+error+message