Yeah, just changing the extension won't change the format the image is in. Most image viewing/editing software should let you do it, load the image up, then do File -> Save As and look for a type in the drop down...
If ya' can't get it straightened out, feel free to e-mail the image to me and I'll get it converted and sent back to ya'...
Any number of us would be glad to help, but Serk's directions are pretty straight forward.