Typically, when you open an image in a post and go back, it returns to the previous page.
However, it depends on how you're opening the image, do you just click on it?
Also, what browser are you using?
I checked a few, but having sample links would help.
If members upload images directly to MMO, it works perfectly.
But if they're using an external image hosting site, clicking on the image opens that site, and there's no option to go back from there.