These can be images that you get from:

Your own personal camera using droplr or snagit
Screenshots that you take
Wikimedia.org
Sxc.hu
CC0 (Create Commons Zero Sites, free to use in any way you like)
Pixabay.com
Unsplash.com
StockSnap.io
KaboomPics.com
Magdeleine.co
Paid photos (that you buy via sites like Fotolia.com)
Just remember that content you get from website "royalty free" websites is not free to take. You need to read the permissions for each image that you use as each one will have different rules.

The Library of Congress https://www.loc.gov/
123RF https://www.123rf.com/
BarnImages https://barnimages.com/
Burst (by SHopify) https://burst.shopify.com/
Creative Commons https://creativecommons.org/
Death to the Stock Photo https://deathtothestockphoto.com/
Epicantus https://epicantus.tumblr.com/
Flickr https://www.flickr.com/
Foodies Feed https://www.foodiesfeed.com/
Freerange Stock https://freerangestock.com/
FreeImages https://www.freeimages.com/
Freestocks.org https://freestocks.org/
Gratisography https://gratisography.com/
Good Free Photos https://www.goodfreephotos.com/
KaboomPics https://kaboompics.com/
Jay Mantri https://jaymantri.com/
Libreshot https://libreshot.com/
Life of Pix https://www.lifeofpix.com/
Little Visuals http://littlevisuals.co/
NegativeSpace https://negativespace.co/
New Old Stock https://nos.twnsnd.co/
Magdeleine https://magdeleine.co/
MMT Stock https://mmtstock.com/
Morguefile.com https://morguefile.com/
One Million Free Pictures http://www.1millionfreepictures.com/
Pexels https://www.pexels.com/
Photo Stock Editor https://photostockeditor.com/
Picjumbo https://picjumbo.com/
Pick-up Image http://pickupimage.com/photo-sharing/top
Pictography https://picography.co/
Pixaby https://pixabay.com/
Public Domain Archive https://www.publicdomainarchive.com/
Reshot https://www.reshot.com/
RGBStock https://www.rgbstock.com/
Scx.hu https://www.scx.hu/
ShotStash https://isorepublic.com/
Skitterphoto https://skitterphoto.com/
Snapwire Snaps https://snapwiresnaps.tumblr.com/
StockSnap https://stocksnap.io/
Stockvault https://www.stockvault.net/
StyledStock https://styledstock.co/
Upsplash https://unsplash.com/
Wikimedia.org.