I whipped up a fun little placeholder image service called Wildcard.fun.
The idea was to make a service that would generate any size image you wanted as a placeholder and size/crop the image to those dimensions.
For the basic structure of the application, I used vanilla PHP in combination with a fast router called "PHRoute - Fast request router for PHP".
In order to resize, crop, and compress the images based on the request I used ImageMagick for that.
The images come from Unsplash.com as well.
Give it a shot!