(DISCLAIMER: Please read the TOS of each free image site to find out the respective usage terms before using an image in your ads. Neither myself nor STM can be held responsible if anyone infringes on usage rights or copyrights in any way.)
Just chanced upon this amazing list of public domain image sites:
https://en.wikipedia.org/wiki/Wikipe...mage_resources
I've used a chrome extension called "Link Klipper" to extract all the links into an excel file:
https://www.dropbox.com/s/q5t37rmlzd..._2019.csv?dl=0
Moreover: I've compiled a list of free image sites for general, non-niche-specific free images. This list is a combination of 3 sources:
-Matuloo's list in this post: https://stmforum.com/forum/showthrea...s-2017-Edition
-Shishev's list in this post: https://stmforum.com/forum/showthrea...lots-of-links)
-Sites listed in the wikipedia page above under the "General Collections" section.
The list is here:
https://www.dropbox.com/s/z6lptg59ns...sites.txt?dl=0
(EDIT: Yet another great site has been suggested by @maynzie here.)
Secret Tip: How to Search for Images on ALL Sites at Once
Tired of browsing to EACH site to type in the SAME keyword and repeating like 80 times, every time you're looking for images?
Here's a shortcut I built using Google's Custom Search Engine, that will allow you do a SINGLE search to find pictures across almost all the sites in the list...
-------------------> https://cse.google.co.uk/cse?cx=0127...11:ik8wz27r1y4 <-------------------
Try it out now! Type in any keyword, click "Enter", then click on the "Image" tab - and the engine will return relevant images across the list of sites I've put into the engine. 
If you want to build your own custom search engine, to search YOUR custom list of sites, here's how...
Step 1: Compile a list of sites you want to search on, on a regular basis. Example: The list of general image sites in my text file you downloaded above.
Step 2: Put an asterisk (*) at the end of each domain (after the backslash of course). You can either do this manually, or in excel by using the "concatenate" function. Example: https://www.pexels.com/ becomes https://www.pexels.com/*
(Note: You may need to tweak some of the urls to make them 'work'. E.g. Change https://commons.wikimedia.org/wiki/Main_Page to https://commons.wikimedia.org/*. Another example: For searching flickr images that don't have copyright restrictions, you'll need to use this link https://www.flickr.com/search/?text=&license=7%2c9%2c10, so won't be able to enter this into the custom search engine. This step will take some figuring out, as every link is different.)
Step 3: Proceed to Google's Custom Search Engine and sign in: https://cse.google.co.uk/cse/
Step 4: Copy and paste your list of sites one by one into "Sites to search". (A new empty field will appear the previous is filled.) Specify the language you want to conduct your searches in. Give your new search engine a name (e.g. "General Free Image Sites") and click "CREATE".
Step 5: You should see the message "Congratulations! You've successfully created your Custom search engine." Click on "Public URL" and bookmark it. Then click into "Control Panel" and toggle "Image Search" to "On".
Then, every time you need images, simply browse to the url, do a search on a keyword, click on the "Image" tab, and you'll see all the results the custom engine has found for you from searching all the image sites.

Enjoy! 
Amy
Amazing Amy, thank you for sharing this.
Oh nice, you really took it one step further and the google custom search tip is very useful too!
Boom!
Super useful Any, thank you so much 
Do you think it can work with paid image sites? like https://www.shutterstock.com/? (if we have subscription plan)
Great share Amy, another nice one if you're a fan of illustration images (You should be if not... they perform GREAT!)
Wikihow + keyword, you can get quite a lot of great cartoony/illustration pics of your desired keywords

Thanks for the share. But not getting a lot of results. Just saying, maybe the CSE is not set up properly.
Example here: https://screencast.com/t/RdtheNoam2