Maybe this will help:
First, though, I haven't been able to directly insert an image stored on my computer but rather had to upload the photo I wanted to an internet album (I used my own MySpace page) because of the way the [img] function works (needs URL). If you find an image on the internet that you want to use...get the URL and use that. Don't know how it works on a Mac but with a PC, right click on the image, select Properties, copy the link showing in "Location:".
Once you've got that, click on User Control Panel (above)
Select "Profile"
Select "Signature" (side tab)
In the space provided, click the "Img" button and you'll get this: [ img ] [ /img ] Between the two blocks, insert the URL of the picture you want to use for your temporary avatar. Like this: [ img ]
http://whatever.ext [ /img ]