Briefly, go to your gallery and select the image you want to post. Click on the magnifying glass above the right hand corner of the image to enlarge it. Once you've got the big version open, right click on it and select properties from the drop down menu.
Depending on your broswer the wording might be slightly different but in Internet Explorer there is a section that says Address (URL). You can select the address and copy it (either using a right click and copy or by pressing "Ctrl" and "C" at the same time).
To insert the image into a thread, type in the following:
[img]paste the address you copied earlier in here[/img]