Sorry for the bump. Just wanted to note the following both for future reference (in case that anyone is searching for the same) and to show some love for the constant work from the folks here at Discourse:
On Discourse 2.3.0.beta6 Release Notes a new feature was announced:
While this doesn’t divide the file sizes or restricts the uploads per sé as in the original request, it does help a lot identifying bigass images. Normally due to scaling the best way to fit an avatar within the limit size is to uploaded it in GIF. While the rest of normal “drag and drop” images just go in PNG, JPG or whatever the user pulls.
With this it is easier to allow some opening while cleaning once in a while by size, even though it may break some posts 
Just wanted to add this reference to the pot, thanks guys!