I still find it amazing how often we see photos on websites which have been illegally lifted from commercial stock photo sites. Sometimes it’s really obvious, because (incredibly) they have some sort of watermark on them (did nobody notice?). Other times I recognise the photo and know it’s one that should have had a credit next to it.
You’re seriously asking for legal trouble if you do this. It’s not hard for online photo libraries to crawl the web looking for copies of their photos. And it’s so unnecessary, because there are plenty of sites which offer images which can be used – even on commercial sites – for free!
Here are a few to get you going. What you’re looking for is a ‘CC0’ licence.