Please, PLEASE resize photos to a good size. Unnecessarily huge photos that fit on nobody's screen and take a long time to load are just annoying. And it's so easy to shrink them! The popular TinyPic for example simply offers it right in the upload form so there's no excuse to not use it:
Their "Message Board" size of 640x480 is probably best for most pictures here. Go smaller if that's still large enough, go larger if you really have a good reason, e.g., it's a sharp high quality photo showing small details of an interesting complex puzzle.
Also, using the [spoiler]...[/spoiler] tag might be a good idea if you want to show many or large pictures. Then they don't make the page huge and people visiting a page again just to read a new reply don't have to load all pictures again. Notice that I do put the below examples in spoiler tags but I didn't with the above picture showing the TinyPic form. That's because it's just one picture and it's quite small (both screen size and file size) so in that case, it would be unnecessarily annoying to spoiler it and to require people to open the spoiler. Plus, spoilering takes some screen size itself. Whether you should spoiler your image(s) depends on the situation, it's a trade-off, just use some common sense. Same goes for *how* to spoiler, like each image in its own spoiler, or all images in one spoiler.