Seen this happening all over the place...
I was told that is to protect the images from being stolen & at the same time for the threads to load quickly.
But IMHO, i just loose the interest to post when i see a decent image on my system & after uploading it shows a smaller picture of much lower quality.
Posted quite a few threads regarding this, bugged a few mods & senior members as well

That's one of the reasons why i decided to end my 365.
I like looking at quality pics & at times due to the forum and/or restrictions a really good image appears very mediocre.
Especially in a photo forum, i think that's the most important factor.
Even while linking from other websites the forum re-sizes and resamples the images