Lorem Picsum - Lorem Ipsum... but for photos
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.
FreeImages - Find and download free stock photos - all free for personal and commercial use
Placeholder.com - Placeholder.com generates custom placeholder images on the fly.
Dummy Image - Dynamic Dummy Image Generator
BlurHash - BlurHash is a compact representation of a placeholder for an image.