Upload your image to Photobucket, Tinypic, there's heaps of image-hosting sites.
When you click on "Insert Image" (It's a tiny .gif icon resembling the Mona Lisa), Above every text entry space on the forum I've seen - It'll put in the [ IMG ] (DIRECT IMAGE LINK GOES HERE) [ / IMG ] code. (Put spaces in so the code wouldn't process, you should do it without any spaces). Copy and paste the "Direct image link" option that your image hosting site gives you. It's the straightforward link usually ending with .jpg, .png etc.