how do i upload from adobe to a compititon site?

I recently edited a colour photo to B &W to enter a comp but can not find how to upload it to the site. It says invalid file. Ive put the picture onto my desk top, then gone to the comp site, upload from desktop, but it will not. So i saved it in "Local Folder " in photos in win 11, so go to upload again select from photos, open local folder and its not there. can any one help please. I am not to tech minded still learning, as a senior citizen.