Maybe not directly Phaser related, but I just wanted to mention that I’ve created a GIMP SpriteAtlas plug-in to convert different sized images into a SpriteAtlas, or sprite texture.
The plug-in takes all the layers and compiles them into one single packed image. It uses a rectangle packing algorithm to use as litte space as possible. It also exports a JSON, XML or CSS file with the sprite names and coordinates.
It can optionally leave one pixel between the sprites, and also has an option to extend certian sprites by one-pixel-row and/or one-pixel-column. This is sometimes needed to make the sprites align seamlessly without lines between the tiles.
I know there are more tools like this, like TexturePacker Online (free) or Leshy SpriteSheet Tool, but to advantage of an opensouce tool is that you can easily add you own custom image processing needs by changing the Python script.
Let me know what you think.