There are several useful sites to provide placeholder images (in varying degrees of seriousness):

Suitable for BBB (Big Boring Business)

http://placehold.it/

https://placeimg.com/

http://lorempixel.com/

http://dummyimage.com/

http://fpoimg.com/

http://fakeimg.pl/

https://ipsumimage.appspot.com/

http://420placehold.it/

For more relaxed environments

Random images

http://loremflickr.com/

http://p-hold.com/

Celebrities (Past and present)

http://www.placecage.com/

http://www.fillmurray.com/

http://www.stevensegallery.com/

http://placesheen.com/

http://www.nicenicejpg.com/

http://place-hoff.com/

Special Interest groups

http://baconmockup.com/

https://placekitten.com/

http://placebear.com/

http://placebeard.it/

Images for download

http://dummy-image-generator.com/

Now I can finally close my tabs on the browser :)