I am BGG's official honey trap
Or is it possible to directly embed them to the review/session/forum post as you go.
If you press the camera with the icon of the arrow you can upload them directly into the post you're working on (they still go into your personal gallery this way but you don't need to remember their IDs).
Other wise for internal images (hosted on the Geek) you need to get their ID number from the images url and press the camera icon in the post you wish to add them to and enter the id. Additionally the default size is small to make them larger edit the post and put 'large' or 'original' after the id string but before the bracket.
For external images (anywhere else on the internet) you need to isolate the image (right click on image, open image in new tab, etc) and get the full url. External images cannot be resized.
Quote Original Dibbler's post above for a coding example of each.