Graphics: changing a color

I've got a button master which is in black and white and needs to be in navy blue and white.

Poking around GIMP turns up colorify (which puts a colored filter over an image) and colorize (which seems to do the same thing). Color-selecting for black produces an amazing crawling stripey pattern that doesn't settle into anything useful.

Any recommendations?

I remember seeing a couple or three web-based graphics programs, but I haven't been able to track them down.