The warning part :
Using copyrighted material on a website without the right of using it isn't a good idea. It could be fine for several years and you would be super happy, your website would have grown etc. And then on day, in the mailbox, you'll find that letter saying you've used a copyrighted picture for X years, so you'll have to pay x xxxx,xx$ for using it. Be aware this could happen (I've seen that in a french TV show called "La facture").
With that in mind, does a big part of your images came from the same source ? If it's a free images library or something like that, you could look there to find some statement of what you've to do if you use site's images. I've personally seen one asking you to state somewhere on your site that the images came from their site. Finding something similar could give you a good idea of what you should do.
Case 1 : If they came from a little amount of sources, it would be nice to make a single page stating from where they came from as it could be combined with other statement relative to your site's policies, copyright, etc. It would also be cleaner as you won't pollute your entire site with copyright notices (that would be almost the same every time anyway)