Good point.
At least on my forum, the vast majority of pictures are created by members. I suppose if someone links a picture to my site (obviously not a member picture...ie a new GM car, sensitive info etc) , and later someone who owns it asks that I remove it I will.
I am hoping that would be a very, very small percentage of the images on my site.
There a 2 problems with asking members to post pictures directly to my site.
1). It's hard to tell people what to do

They like to do what they want.
Some say it's just easier to upload all their pictures in batches to a place like photobucket then link them when they need them.
2). Hosting the photos on my server would significantly increase bandwidth usage so It's actually preferable to have them hosted elsewhere and just have a link to them. I backed up over 5,000 images yesterday (not all FROM yesterday however).