This was tried once when images were first introduced (I was a mod at that time and running the Travel Photography forum so was intimately involved). The decision made at that time was to not do it and rather to use moderation and the general social norms of the community to encourage images no larger than ~800x600. And, quite frankly, it has worked for years.

If anything, access to more and better bandwidth is increasing, not decreasing, at this point. Ditto for larger and higher resolution screens.