default kubrickheader.jpg appears even though its not there

i just started using wordpress and i have a good knowledge of php; however, i cannot figure this one out. Basically i replaced kubrickheader.jpg with another jpg called kubrickheader.jpg. i then went to my site and the old blue header was there. i hit refresh and the new header appeared. i thought brilliant, it works. well i hit refresh again and the blue one came back. doh! so everytime i refresh i switch between the two jpgs. i thought maybe it was something weird with IEs cache so i cleared the history, deleted all the cache files, and tried again. It still switches on every refresh. any clues to why ti is doing this would be great. searching the forum i haev found so much other helpful information but none on this.

thanks