Photo copyrights masters

I'm currently into editing photos, making them look like the ones you see on Tumblr with the text in the middle.
I was paying for photos at first but this is just way too expensive for me to afford.
I stumbled upon Pixabay, and they offer great photos for free. Thing is, they do not offer any kind of guarantee or license, they just assume that
the uploader didn't steal it or something. so my question is- how can I actually verify that the photo is legit and that it didn't get stolen from Istock etc?

Thanks.

You can try using something like tineye.com or googles image search.

Use google image search to find if its already used

You can search on Google Images and check.
If that picture exists there than that was stolen and you are scammed.