I guess it depends on what/how these images are going to be displayed. Are they just wanting an image url so they can post that picture on other sites or what? If so, check out these...
http://codecanyon.net/search?utf8=%E...m=image+upload
I haven't done or used any image upload scripts, but those would seem like the best bet.
If they own some sort of gallery, it would be best to just use a CMS like Wordpress. Create a gallery page that will just allow them to add and delete pictures with the built in [Gallery] tags.