More thoughts on screen captures vis a vis photography:
1. Authorship is an issue even more than with Sherrie Levine/Richard Prince "rephotography," assuming the capture is some one else's work other than the capturer's. Putting a Walker Evans on the copystand, printing, framing, and exhibiting vs hitting the "printscreen" button, making a jpeg, and uploading to a blog.
2. Photography at its most indexical doesn't confuse as to its purpose. Whereas a screen capture that includes, say, YouTube controls, is far more likely to be mistakenly clicked by the consumer than merely passively viewed.
3. Captures are inherently irresponsible. Unless the capturer has included a surrounding frame (such as a web browser's scroll bars and address bar) the viewer has no way of knowing if the capture came from the web, the capturer's home computer, or somewhere else entirely. There are no easy verification methods such as looking for telltale signs of the clone stamp to detect Photoshopping.