Upload the picture to imgur, get its address then use the picture button on the post reply page - just above the first emoji. Put the address between the tags that come up.
This one is from Meteor Strike (mourning):
https://steamuserimages-a.akamaihd.net/ugc/928171807498487568/F3F7216536F61C0359E097B1FA24C2AE3583318A/?interpolation=lanczos-none&output-format=jpeg&output-quality=95&fit=inside%7C640%3A*