Reduce the size of the picture to at least 640x480 pixels. This makes the pic small enough to fit within the constraints of the upload, but leaves enough detail for everybody to see it well. If that is too small for you, try 800x600 (SVGA) or 1024x768 (XSVGA). But don't go larger unless you have to for the detail.
Also, if it is a JPEG, you can play with the compression ratio. JPEG is lossy compression, so don't get too crazy; you'll lose too much detail.
The GlueGuy