Lorem Picsum - Lorem Ipsum... but for photos
FreeImages - Find and download free stock photos - all free for personal and commercial use
PlaceBear - Bear-themed placeholder images for developers
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.
PlaceKitten - Dead-easy placeholder images. Of kittens.
BlurHash - BlurHash is a compact representation of a placeholder for an image.