I don't think there's anything wrong with using a picture of an animal you don't actually own (so long as you own rights to the picture itself) however, it should at least be a species and morph you are currently working with. It can make a BAD first impression if someone comes to your site expecting to be able to find green tree pythons or a mystic potion ball python because you have one in your logo...only to find out nothing of the sort is available.
This is an attitude about stealing internet pictures that gets to me every time I see it. Just because a picture doesn't carry a specific watermark does NOT mean it's up for grabs to anyone who wishes to use it.
That's a dangerous assumption to make. Steal the wrong person's picture and they won't care how big your wallet is, they will go after you hard just on principle. Seen it happen many, many times for many different reasons. "Small fish" are absolutely not exempt from legal action just because there may be no money to fish out of that pond.
Also...and far more important...is your REPUTATION is on the line. Your logo is your face to the public. One of the fastest ways to trash your reputation before even getting out of the starting gate is to get called out in public for using someone else's picture without their permission. That can and will happen fast and furious in the Internet shark tank. Take some time to cruise through threads on the BOI if you doubt that.