No problem. I do not believe it causes an issue for search engines, because when they crawl a site for images, they copy them to create the thumb sized image like on Google image search results, for example.

*But* if you was to click on a search result thumbnail (as on Google), you notice it opens the actual page the image is on and also a frame at the top where it houses the thumbnail again - If you was to then click on this, usually the full sized image will appear, however, if this was a site where they use such preventative measures, then it would come up with an image of the webmasters choice stating that it is a 'hotlink'. So it stops people from grabbing the image in such away, but if they let the entire page download with the image on, instead of clicking on the thumb to just get the full sized image, then it is not a problem.

...Or at least, I'm pretty sure that is what happens, unless anyone can tell you any differently.