“Photos cannot be sold without significant modification”, License Unsplash
“Sale or distribution of Content e.g. as a posters, digital prints, music files or physical products, without adding any additional elements or otherwise adding value”, Pixabay Terms of Service

Photos must not be used for commercial purposes, including as part of a trademark or logo, i.e. for advertising purposes.

“You’re only allowed to use content that you find on the site in connection with your use of the Services and solely for personal and non-commercial purposes”, GIPHY User Terms of Service
“You may not use content as part of a trademark, design mark, tradename, business name, service mark, or logo”, Content license agreement FreeImages

Photo stock website is not responsible for possible copyright or trademark violation when using photos for personal and commercial purposes.

“Some of our images contain products from well-known brands and companies that have registered trademarks. These trademarks need to be respected and acknowledged”, IsoRepublic IsoRepublic License
“It’s possible that some pictures may be subject to other intellectual property rights, such as trademark, publicity, or privacy rights. Gratisography does not represent or make any warranties that it owns or licenses any of these rights or can it grant them”, Gratisography License