I'm curious. If 3d art models are allowed, why not good photos of furry sculpture or plush?
I have seen a few posts taken from Second Life. Is this allowed? If there are restrictions, what are they?

Read the Uploading Guidelines for a comprehensive list for good and bad things to upload.

Photos are ultimately irrelevant on this site, if they were allowed, people will start uploading videos and other stuff (e.g., fursuits, etc).
Second Life stuff is normally not allowed due to them basically being screen captures (see topic #25043), so I'm not sure what the exact criteria is for them to be allowed (other than being grandfathered in prior to the rule).