I wanted to delete a picture that I have in projects, but I can't seem to do it. I must be doing something wrong or missing a step - HELP!

just follow mops (Martine's) answer and you will get it done easily.

Go to 'my account' 'my projects' 'my projects' and you'll get list of all your projects. Look for the one you want to change. Choose 'delete if you want to delete the project. Choose 'edit' if you want to change the wording or the picture. Next to the picture place a check mark in 'delete'. Choose a new picture and submit, and the old picture will be replaced by the new one.
Just tried it out on an old project I deleted afterwards; worked fine for me.