To edit your uploaded photos from the Project Photos page, click the gray "i" icon underneath the specified photo.
There you can choose to "Add/Remove File Label", "Edit Description" or "Delete photo" using the corresponding links. You cannot rotate or crop photos once they are in the site, you will need to do that before uploading them.
For pictures uploaded to selection items, you can also edit these photos by clicking "Edit" on unlocked spec or selection items. Here you will also be able to add/edit photo labels, add a description or delete the photo.