The script is asking for a link (
http://… etc. etc.) this assumes your image is being hosted on the web somewhere at the address you specify).
Talkbass.com also allows you to upload graphics (up to 600x600 pixels in size) which you can upload as an attachment (may be some restrictions dependant on the type of membership you have…) see "Additional Options"->"Manage Attachments…"
Hope this helps -
- Wil