I don't believe that was what caused the problems, although overly large images will always slow down the loading of the forum, more for those with slower broadband speeds.
There is a facility to limit the size of images that can be uploaded, and we would use it if we needed to as a last resort. However, it seems to be fraught with potential problems - the limit has to be given in kB rather than pixels, and there is no way of saying that a picture of a certain width/height will be OK, because there is no direct correlation. I don't like to think of the reaction if we told people they needed to adjust the kB size of their images before uploading them!
I have also heard reports that rather than being told that the image is too large, a user is likely to get one of a variety of error messages, which will make them assume the forum is at fault and not them. More flak for the admins.
However, as phone cameras are now taking truly enormous photos, we may have to try using this facility at some point.
Much better if people just think before they upload though.