Cloud storage options for iPhoto's primary storage (not for backups)
Your best bet is to wait for the release of Photos for OS X Yosemite, which is going to replace iPhoto early next year and will do exactly what you're asking for.

You'll be able to store all of your photos in iCloud, and the Photos app will dynamically cache only the most recent and/or frequently accessed photos locally, freeing up lots of disk space.

Pricing will be 99¢/month for 20 GB or $3.99/month for 200 GB (or 5 GB for free, but that's really not much). There's also going to be a 1 TB option, but Apple hasn't announced pricing yet. 

OS X Yosemite and the new Photos app will both be free updates, and will run on any MacBook Air released in late 2008 or later (which is every model other than the very first one).