First you'll need to have the photo that you would like to upload.
Then easliy go to http://tinypic.com/
and choose your image that you would like to upload, enter the matching code.
Then copy the direct link, have fun.
There are many websites, but the best for RC-RP uploads would have to be Photobucket. Google it, Sign up and you can have unlimited uploads, and it already has a Copy and Paste link for RC-RP, AKA you can copy the link and use it on RC-RP forums.