Find an image hosting site that you like, such as Imageshack, Photobucket, Imgur, etc, and make an account on it.
I use photobucket so I'll be using that for the example. Near the top of the website should be an upload button, click it, and click "Select photos and videos". Select the image you want to use (if it's a BL screenshot that's "documents/blockland/screenshots", or similar on all windows (no idea where it is on mac), and upload the images. Next go to your ablum, scroll over the image and click on the "IMG Code" box. That copied it, simply paste it into the post and tada!
The img code is at the bottom of the box that pops up when you scroll over the image and the code looks like this:
[ IMG]http://i413.photobucket.com/albums/pp217/sord213/Blockland_00005-1.jpg[/IMG]without the space at the front it's a normal image
