If you have Microsoft Office installed, right click the file and select "Open with... Microsoft Office". Then under the Picture menu, select "Compress" and set to Web and save it. That will usually get it small enough to upload.
Another way is to upload the picture to some Internet site then link to it in your posting using the Picture button up top. I use Picassa to upload. If you do that, you don't have to compress it first since Steve's site doesn't load it, just displays it using the link.