Two ways I do it.
If I have the picture on my computer, I click on the icon in the bar second from the right above where I type the message. It brings up a box where you can click on Browse. Click Browse and find the picture on your computer click on it and then click on upload which is on the line below where the link appears.
The second way I do it is by right click, select copy from a screen where I found it, in this case eBay then go back to forum and right click and select paste. This doesn't work in all situations though.