When you create a Google account, the company gives you a set amount of storage for you to use in Gmail and other products.
But if you’re using a backup product like Google Drive, you can actually buy more.
To do so, head to the Settings app, and then click on the “accounts and import” tab. The very last link should say “upgrade your storage”, which will take you to a new web page.
There, you should be able to buy more storage for Google’s online products, including Google Drive and Picasa. It’s cheap, too – only $2.49 a month for 25GB.