Finding good sources of stick images is difficult, and as a result many small businesses resort to Google image searches which means that frequently they end up using images unlawfully and can end up facing big legal bills.
At the moment my go to places for stock images are as follows.
My current favourite is pexels.com offering a large selection of images under the Creative Commons Zero (CC0) license which means anyone can use them for for any purpose.
Another regular site is pixabay, which is very similar to pexels in the way it operates.
More CC0 images this time brought to you by Snappa an online graphic design package.
This one is more like a gallery of created images rather than your traditional keyword search but offers amazing royalty free images non the less.
This one takes a little more effort but by using the advanced search tool on the flickr website you can search by licence allowing you to find cc0 images.