If you cropped the picture in the Photos app, Dropbox will upload the uncropped picture. When you prefer the cropped version, use a different app to crop it. Snapseed, Skitch, Photogene and others will crop your photo and save it to the Photos app, from where Dropbox will be able to upload it.
Dropbox won't load a picture for a second time, if it thinks it's already in it's storage, even if you deleted it there.