vBulletin has that feature, on the forum I'm an admin on, we limit the size of images to 1024X768, more because many of our members are in 3rd world countries and have limited bandwidth, than because of the annoyance of overly large images.
To show what Corvette Joe said, look at the screenshot.
Edit: THat didn't work the way I expected, on the forum I'm an admin on, there should be a clickable thumbnail, that should open a larger version.
I'm an admin on another forum, we actually have in our code of conduct that telling someone to search for an answer themselves is not an acceptable answer. Our biggest problem is we are seeing many new members (we have close to 2 million members) that don't seem to want to do any of the work...