To take a screenshot, use the Print Screen key located to the right of F12. The file is saved to your Warcraft III/Screenshots/ directoryPlease remember to convert your image to a .jpg before uploading.
If you forgot to save the replay, upload the file 'LastReplay.w3g' in your Warcraft III/Replay/ folder.
Depending on your connection this could take several minutes.
Download a .tga to .jpg converter here