MOG
I successfully posted a pic on the Private Flyer forum after reading the following.
Stik (ex aircrew)
First you need to upload your photos to a website. Some websites allow you to link to them - others don't.
I use fototime which lets you store some pictures free for 30 days and link to them. I'm sure there are lots of other suitable sites.
Once you have done that, click the button marked "IMG" when creating a message, and paste in the url of your photograph. (Easiest way to do this is to right click on the photo on the website that is hosting it, select "properties" and copy the address.) If you use fototime, you can instead click the button marked "create link" and copy the address from the next screen.
It sounds a bit complicated, but it's easy once you've got the hang of it.