Best way is to set up an account at www.photobucket.com . when you upload a picture it will give you a URL (http reference) to your photo.
Then.. when you are writing a post, click on the image icon, and a little javascript box with http/ will appear.. cut and past the URL into there, and hey presto you have a posted image...
Some of the forum sections allow you to upload images instead, but not all... so i find the above the easiest way.. that way, if you remove a picture from photobucket, then you can also be sure that any posts referencing your image will also have the image removed..
Hope that helps.. it doesnt take long to sign up, and once youve done it you will have a place for all your photos to refer to anywhere else too..