Backgroundifier - Turn oddly-sized images into beautiful desktop backgrounds
ImageKit.io - Instant multi-platform image optimization
Gumlet - Gumlet is an end-to-end video streaming and image delivery platform
Promo Instant Image Resizer - Automatically optimize images for web & social media (free!)
FreshBackMac - Fresh backgrounds for your Mac from multiple sources
Promo.com - Every promotion starts with Promo.com.