I've always used Photobucket, which seems to automatically post the pictures at a sensible size. I think the trick is to use the link tagged 'IMG' for forums.
Here's a direct paste of that sort of link, and an excuse to show off Henry's Christmas bow tie! It comes out as the right size automatically.
![](http://i1104.photobucket.com/albums/h325/rachaeldunlop/IMG_1038_zpsji0qe455.jpg)