phpBB had this and was a cool hack. Members post an image hosted elsewhere using the [IMG] tags, however the size of the picture exceeded the width of your forum, throwing off your fixed column widths. This hack would then quickly resize the image for viewing in the thread and made it so that if you wanted to view the image full size, click on the image and a new window would appear with the image at 100%. Been looking for something similar
here, however it seems that nothing close already exists.