The best way is to put the new picture in the comments, but be sure to say that it is a replacement pic.

What title did you submit pictures for? Was it a TV Series? If someone is working that series, we may delete other pictures submitted to allow the regular poster to add their pictures first. Once that poster has finished adding that episode, we would then allow other posters to add new pics. It's not a written rule, but a common courtesy.

We don't have PM capabilities on the main board, just the forum.