I found a photo online of a girl who looks eerily similar to my girlfriend but she claims it isn't. Can I prove she's lying?

No... everyone has several people that look eerily similar to them.

What type of "proof" are you looking for? Without too much info what kind of site was the picture found on?

Have you even asked your girlfriend what she thinks?

I once had a guy walk up to me in a coffee shop, address me by my first name (correctly) and proceed to chat away for at least 10 minutes before I realized that there was no way I knew him and that he thought I was someone else - and we share the same name!
I eventually met her and although there were certain similarities, I didn't think we looked alike enough at all to mix us up.
The only arbiters are the 2 people in question.

Too many people with the same name even look a lot a like. Unless you have too much that adds up for you to pursue you need to just drop it before you ruin your relationship

I think the only way you could prove this would be by verifying other things in the picture as well.

I see this relationship ending in the near future...

Why are you with her if you don't trust her? She says it's not her, and you want to prove that she's lying? Just break up with her now and save yourself the trouble, because that's what will happen if you continue trying to prove that she lied.

But, to answer your question, there's no way to authenticate that it's a picture of her, especially if it's not, which is what she told you.