Being a forum website owner myself I know that pictures use up a lot of bandwidth. I guess it is up to the forum hosts as to what pixel-size photos will be allowed.
Many pictures that are pre-formatted in your picture-viewing client on your own computer will show them automatically as a fixed percentage of full size but if the size has not actually been cut down for internet use they can finish up being massive files.
E-Mail clients are the worst offenders here as they almost always show the picture at full size, ( the size and extention used when sending them ), and then often a full expand of the picture is not enough to see it without scrolling the page upwards and sideways.
I usually resize them to be around 600x400 pixels and ensure that they are in a lossy format like .jpg. This is usually acceptable by most forum hosts.