The easiest way to insert photos from your gallery into a forum post is to use the My Pics option in the Advanced forum post editor mode. You can find it under the clickable smileys to the right of the main box:
Note where the 'hand' mouse pointer is, on the 'Open' link.
Click there and you will see a pop-up clickable thumbnail gallery of your gallery images, in date order, with the newest shown first:
First ensure the message edit cursor is positioned within the message edit box and then click on your picture of choice.
There are various display options, but I'd recommend 'Original Image' and 'Centered'.
If all goes well, your picture will be inserted into your message a bit like this:
To double check, use the 'Preview Post' option. You can repeat the process to easily and conveniently insert as many pictures as you like - within reason!