A small, lower resolution image seems ideal. Anyone who's even considering buying a full-res photo isn't going to see a 200x300 image and say "what the heck, I'll just take that instead." If they did, they weren't going to buy the original either.
And if you want to give the users a sense of the final resolution, you could take a corner of the image and show that at full-resolution, while making it clear in the UI that it's only a corner.
I do agree that a watermark is (strongly) preferable for choosing which individual images to buy - small details can make or break images, and this obscures a lot of the image. Just pointing out that there may be uses on the same kinds of sites.
For those who have turned meme-creating into a sport, watermarks and other attributable data are vital in seeing how that meme spreads. I know people that deliberately put a yellow 1px border beneath each image they share (and created) so that if they spot it somewhere else, both they and others can guess where it originated. There are other more sophisticated methods which I can't mention here for detecting meme spread, but watermarks are a big deal in the meme world.
I found that very funny in a austin power kind of way, like this is some sort of giant secret technology.
The most efficient methods of watermarking pictures in ways that are easy to detect, easy to hide for the viewer yet hard to remove for the copier are all well known and detailed on the internet, in fact wikipedia is a decent source before going into more technical content.
And in terms of "memes", this sort of watermark is most often done by individual users for reasons related to ego rather that copyright protection. Because the actual money making memes oriented websites will prefer the good old heavy duty watermark all over the picture, since it will be copied anyway they figure they might as well ensure the final viewer knows where it came from so he can look for it and get them new traffic.