Question 1: Your knowledge or impression of the source. References? Quality clients? Clearly you have a concern; a reason to question.
Depending upon how the image has been edited, there may still be EXIF information. It won't reveal much to answer your question, but may still contain data on camera used, and the creation date. Might find a lie. Long shot.
Since it is only a couple of images, then I would do an image search on the likeliest keywords/file-names and see if they pop up in the results. If you asked him to find an 'apple tree' image, it is easy enough to run image searches and find you some nice apple tree pictures to choose from.
This is a flag for me - I would really want to know why this small image size is all that is available? Did they take the pictures? If so, why nothing more than minimum size available? If not, you want a release from whoever did. Maybe they should only be getting a middleman cut and you deal with the photographer yourself.
If I were going to spend money, I would spend it on stock photo subscription and then no worries at all. Plus, if you are buying web presentation images, as opposed to 'full rights', high end print use, and such, then you can probably get a petty wide and solid selection of images at a great price.
If any doubt, there are tons of images for sale at good prices that are clean and lots of legitimate sources to choose from.
If you do buy from him - DOCUMENT exactly what you are buying and his right to sell it.
If you are still 'ill at ease', but take your chances anyway, don't name the file 'apple tree' or give it a good alt description. Then the image bots pick it up, post it, the real owner does a search every by-and-by and one day he finds his images on your website. Oops.
We have caught plenty of people with our images out of complete luck. They don't rename the file, or they give it a name that makes it easy to find again.
Probably you're safe enough if the image is used without context that could get it flagged. Stumbling upon it would be almost impossible. There are services that can 'track' images, but I have been told that they follow robots.txt - which makes the service worth less if you Disallow correctly
The bigger issue is if this is someone you have done business with before, will do more business with..... If it feels a little shady, go with that feeling and be careful in proceeding.