Multiple links, especially to large images, in a single post will slow things at the
server end, the forum server has to process the links, fetch the images from the
site, place them in the post, generate the post etc. The traffic and speed at the
source server will also effect the process.
Multiple posts are the better option, especially as, along with the wait for the post to
load, scrolling forever through a single post can be both irritating and difficult for people
on slower devices.