^^^ Theoretically it would be ok to have links to external hosting, in -practice over time- it creates havoc on any forum.
Apart from things like the Photobucket scam\extortion with millions of photos disapearing overnight all over the world, people don't remember where and when they have posted picture links in forums, then they do modifications in their photohosting -or change to another hosting or close their account = all of their posted photos disapear all over = destroying important information and making forum posts useless.
I hate this, specially on tech related forums where lots of the info, value and insight comes from photographic material.
Am seeing this photo disapearance on so many electronic \ tech \ mechanic forums I frequent, it's an internet catastrophy really.
As for storage on the forum, it's zero problem, -vast- amounts of space available (so far using 8GB (incl. some backups!) out of 125 GB available). Storage is very cheap these days.
The picture size restrictions is not about server storage, but relates to good size for viewing on screen + decent loading times for members\visitors, some are on dial-up\satellite\low speed mobile\metered with no access to 4G or fibre etc, 15 MB 6 x 4 meter large picture are -not- ideal in practice :O)
And making regular backups of everything (copies also available to other forum staff\moderators) so whatever happens the forum can be reinstated on any hosting\server in any part of the world at any time with pictures and content intact, secured for the future.
Using external picture hosting = sooner or later important content and information destroyed.
Thanks
e.