I try to only use my own photos.  I also occasionally will use something from Wikimedia  commons or the public domain.

I have used pictures that seem to be free by googling free clipart.  There is one company that continues to try to shake me down for money for having used their pictures.  I ignore them but took those that I could find down.

I'm now going o use https://pixabay.com/ which was suggested by:

Tony and Suzanne,

Our photos, and we do buy photos.  Some Wikipedia photos that are in the public domain are free, others need photo credit.  A

Flickr - make sure to search for those which allow commercial use for free. Follow their instructions and give them photo credit.

Hi Tony and Suzanne- my best source for photos is my own iPhone or camera. The site I use the most is iClipArt but I pay a small annual fee. 

Occasionally, I use freedigitalphotos.net but usally I use my own photos and graphs.

I either take my own photos or draw my own clip art.

I have used clipart.com and paid an annual fee. I have used freedigitalphotos.net but give attribution.

Great question. I try to mainly use my own pictures, but there are times when you need something you don't already have. 

I buy from 123rf.com and I Google royalty free photos.

My camera.

I use Google Images and filter for free to use items.  Other than that, I'd just do a google search for free clipart.