I think there is a guide somewhere in this forum, but I simply copy the BBcode from Flickr (in the Share pop-up). Works like a charm. I prefer larger images, as they are automatically scaled in this forum and look great on Retina displays.
1. please make sure you use BBCode
2. It's a good idea to size for forum display. I use 1024px on the longest side. People can click through to a larger image if they wish. Posting larger images can lead to sideways-scrolling - which tends to piss people off - and can make threads look and behave oddly. Forum software (both iForum and Tapatalk) will also try to compress oversized images with often undesirable results.
As an aside, it's also a courtesy to others to remember that not everyone is on a fast or indeed an unlimited connection.