Yeah, the .gif is 6.6mb on disk.
The forums do not support images that large, as another user already noted.
Okay, I'll come clean. I pulled the 2MB limit out of my butt! Honestly, I did some limited testing a while back and it seemed that 2MB was the sweet spot but as it turns out, Theymos had originally set the limit to 1.5MB and then in December of 2013, increased the limit to 2.5MB at the request of
bitpop.
REF:
https://bitcointalk.org/index.php?topic=388403 So the limit is 2.5MB - fill your boots!