Many boards, like this one, place a limit on the size of graphics files they will allow. The obvious solution is to reduce the size of the file. There are several ways to do this, and here are a couple of them.
Find out the resolution of your picture, which is usually listed as dots-per-inch (dpi). If your graphics software will permit, reduce the resolution. Example -- if the file is at 300 dpi, reduce it to 150 dpi.
Find out the dimensions of your picture, usually expressed as X units by Y units (typically inches). If your graphics software will permit, make the picture smaller.
Or, you can do a combination of the above.
The critical point here, of course, is what your graphics software will do for you. I happen to use a Hewlett-Packard package, and of course there's PhotoShop.
Hope this helps!
-- Nighteyes