I've found why you can't post your photos.
It is a security issue. The way that Shutterfly and many of such sites store your images would -- if posted here -- allow the site to be hacked. It could allow someone to run a script that would let them get into the site and take it down or deface it.
This is why this ability was changed last year, many versions of the software ago (2.0.8; this site is running the current version, 2.0.13).
Right now you can post images with the following file extensions: .jpg, .jpeg, .gif, .png
You'll note that Shutterfly and other web album site have no photo extension at all. This is because they 'deep link' to the site -- bypass the login and other parts of the site to link to a file that is several layers down in the site.
So there is no way around this but to upload the files you wish to share to a site like flickr.com (my favourite) or photobucket.com or muchos.co.uk.
Be aware that if sites like Bart's update their software (which should have been done and probably will be done eventually) all the Shutterfly images will disappear off the site.
If it is possible to link to the *actual image* on Shutterfly rather than an album photo -- eg to a pic that has one of the file extensions noted above -- it should post. I don't know if you can do this at Shutterfly.