The problem with the yahoo photo attempt and imageshack attempt is apparently you are trying to link to photos but instead are linking to the website pages - not the actual photos. You did a similar thing with the photo housed on the kingsnake photo gallery. The one photo that did show up was linked properly. The other photo was linked to the gallery page not the actual photo.
The printroom attempt didn't work as that apparently requires a user login and then you might still be trying to link to a website page and not the actual photo but I can't tell because it keeps asking for a log in.
Another one of your attempts was to try and link to a photo on your harddrive... impossible in this case as the forums only allow photos that are already uploaded some where on the internet.
I could not find the image url on the yahoo site to make your photo work on the forum. It's possible that yahoo doesn't provide them as they don't want people cross linking photos from their site to other sites. Some image hosting sites restrict that stuff for some reason.
For the imageshack attempt, the link you used was: http://img177.imageshack.us/my.php?image=1002989pe4.jpg
but the image url you needed to be using was: http://img177.imageshack.us/img177/705/1002989pe4.jpg

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PHWyvern