There are various sites that provide free images. Some give you the tools to create the images yourself, and some provide royalty free or creative commons images for you to use.
We have bolded the ones we recommend for quality of images and ease of use.
NB: If photos have people on them, there may be a model-release involved. Please check for these.
PhotoGen - amateur high resolution photos, free, no strings
Pixel Perfect Digital - free high resolution professional photos, extra options such as colours sometimes available, attribution information shows when downloading
Freepik - nice graphic designs, icons, careful of clicking to premium sites, files in Adobe Illustrator format (only for people with graphic design experience)
4freephotos - free photographs, diverse range, a lot of ads, can be difficult to browse