Puzzle of the week: Nonograms

Thanks to my puzzle friend in mysterious land, I have a new entry in my collection of delightful time wasters. Nonograms are number-based logic puzzles that can be arranged to display a hidden picture when solved. The puzzles are known by many names. If you lived in Japan and owned a GameBoy, you might know it as Mario Picross!

The default 5x5 format is good for learning the rules. After that, you can choose from larger game boards, and it gets a bit tougher. Can you solve a 10x10 grid? 15x15?