The generic placeholder image in your contacts app for people lacking a proper photo is pretty boring, isn't it? Ideally, you'd want to add a photo of each person, but maybe you don't have one. A handy alternative is Micopi, which generates cool geometric designs in place of the boring old silhouette.
Micopi isn't just using a random number generator to create the images for each contact. It bases the image on the contact's phone number or email to generate a pattern. This all happens on the device, so your data isn't going anywhere. The result is a unique, colorful image that looks nice in the contact app. Micopi lets you apply images to individual contacts or to all of them at once. The app produces images with a sort of material vibe. It might not be everyone's taste, but I like it.
Be aware, there's no way to bulk undo this process. So, maybe you should just generate a few contact images to make sure you like the look before you're stuck editing your contacts individually. There's also a bit of artifacting visible in the full-screen version of the images using the Google contacts/phone app (see the far right screen above). That's probably Google's fault, though.