Dear Aunt TUAW,
I have a MacBook Air with only 120 GB of storage. My iPhoto Library is over 40 GB. I don't need all the the photos on my Mac but don't want to lose them or the events that they have been sorted into. Is there a way to keep my iPhoto library stored on iCloud and only download the albums that I need?
Your loving nephew,
Flickr Pro costs about $2/month and gives you unlimited uploads and storage, ad-free sharing, and a few other nice benefits. Uncle TJ points out, "As much as I don't have confidence in Yahoo's management of Flickr long-term, there are enough people who have pictures there that the ecosystem is rich enough for a lot of 3rd party support systems."
Although you could probably use another service like Dropbox or Google Drive, Flickr offers specific photo management tools and doesn't cache your data locally the way Dropbox does.
Auntie acknowledges this isn't exactly the answer you were asking for. Of course, with Auntie, when it is ever? She answers the questions readers should have asked, not the ones they actually do. With that in mind, she and Uncle TJ recommend Flickr Export for iPhoto, which does a whole lot more of managing how iPhoto syncs to Flickr, than iPhoto might do with its built-in functionality.
Basically, this is the solution that Auntie would use if she had to manage pictures on an MBA without a secondary computer.