Put your image somewhere on the net and then link to it. For example, if you can FTP your image file to www.TuPics.com or wherever, then you can link to it using its fully qualified URL address.

For example, if the image file you upload is named "www.tupics.com/images/button1.gif", then you will simply link to it using that name. Be sure to prefix it with "http://", so that the full thing you type in is "http://www.tupics.com/images/button1.gif". The required format is very specific, so the "http://" and the full and complete URL address is very important.