A lot of the web hosting sites like Photobucket will allow you to automatically resize them to bulletin board size when you upload them to your site. You also have the option of uploading them as the full size file. Generally speaking that is the easiest way to do it. Just check your options menu on your web hosting account.
if you upload to imageshack.us, there is an option for "thumbnail size" where it automatically gives you a link for a thumbnail that links to the page where you can then click to see the full-res image.
^^ Something sent to me to be touched up in photoshop