Backstory: On most teams, I use dropbox to share and transfer images. Often times designers or marketers will load full, raw, 5M+ images in there and someone is bound to use them on the website. Losing conversions and traffic due to long page load times is frustrating.
The Idea: A dropbox extension that automatically minimizes and shrinks images.
Deep Dive: You could build this a number of ways but offering this as a one click service would be huge. You would build a platform that authorizes access to a user's dropbox and creates a "minified" subfolder in every folder that contains images. You would give the user options to exclude directories and tweak image settings.
Validation: I do this manually now with tinyPNG and it works but it's a hassle.
Monetization: If this is done automatically you could offer this as SaaS and charge on a volume basis. This would scale with larger agencies using your platform.