If you run a windoze PC there is another way to what DG said too. Get what you want on the screen, then hit the Prt Sc key. Open up "Paint" and then hit ctrl-v to paste it onto the page (or right click, then paste). Crop out what you don't want and save it to a location you remember.
Then, in a forum message, click the icon at the top of the text box with the mountain in it. Hit browse in the pop up and find where you saved your picture to. When you've found it, double click on it. Then click the insert image box to paste it in here. Don't resize it if it's got text as it becomes hard to read.
