You should be able to do this now but the issue is that large images are resized to fit into the post box. Otherwise the stretch the forum skin sideways and things break. Currently large images are resized but you can click on them to enlarge.
This is a balancing act - while some view on nice large monitors, others are viewing on tablets or smaller devices - finding something that works for all platforms is challenging.
Here's a big image that has been resized by the software - click to expand..