For their own emotional sake. I just saw a really heart-rending Facebook post from an acquaintance who used a flattering pic and lived to regret it when her date left her feeling like dogshit with some tactless honesty.

Whatever benefit she got from the pic that was better than reality I think was reversed by the breakdown she's now humping through.

So the moral is, stick out your gut more than it usually goes. Wear exactly the worst color for you, exaggerate your extra chins.

If the dude ends up meeting you, the look of pleased surprise on his face will be like my friend of a friend's experience in reverse.

TL:DR: manage expectations smart and you'll be glad you did.