I was thinking of making some of my better photos available as stock for Phenomenites for layouts etc. but I have no idea how to tell if an image would be of good enough quality for use.

Any hints?

Well, you can always run it by a graphic artist.

But usually I look for focus on the animal for starters, and lighting.
For backgrounds its 

"Is it big enough to look natural?"

Thanks!

I guess I could post some and if people want to use them, they can. ^_^

Exactly!
If they don't, no harm!