The use of stock photos vs original photos has always been debated in SEO. There are some that say original photos are best and can get you ranked better. There are some that say using stock photos do not matter.
In a tweet from user @seomcgarry, John Mueller was asked if Google cares if he uses the same image from Unsplash that 10,000 other sites have used.
Mueller responded that if you are focusing on web-search, you can go for stock images. It is totally fine to use stock images. However, if you are trying to show up in Image Search, stock photos make it harder since Google would try to just show one of the copies of the image. However, this should not cause much issues since many areas or topics rarely use a visual search.
Looks like the use of stock images is not a big deal and is not bound to affect your chances of ranking in search.
SEOIntel has also tested Original VS Stock Image in one of their earlier tests. In the test, the page with the stock image beat the page with an original image, showing that using stock images does not really matter and will not put you at a disadvantage against original images.
If you do not have the budget for original images, then the use of stock images should be of no problem. Note though that there are some use case where original images would still be the best, not ranking wise, but more on the conversion and user experience side.
In addition, if you would like to target and get ranked in image search, original images should be used.