Here are a few suggestions, and we are sure you would be to try these out: Ms.
However, one thing you need to keep in mind is that you should have a 184 x 184 px steam profile photo because this is the standard size of steam avatar.
I am this gallery below which contains 2000+ steam avatars each of size 184 x 184 px.
You can use them in any way you like, as long as that use does not violate our usage agreement.