Well it definitely is in the right file format. Yet it does not provide an alpha-channel. Thats why it isn't transparent. Which program do you use for editing your images? If it is GIMP here is the way to do it:
Option 1: New image

1. Start The GIMP
2. Press File->New
3. In the dialog where you can choos the dimensions click on advanced settings
4. For filling choose transparent
5. Draw you image

Option 2: Adding alpha to an existing image

1. Start The GIMP
2. Load your picture
3. In the layers window right-click "Background" and choose "Add alpha channel"
4. Either use the eraser to delete unwanted areas or use the magic wand, path tool or color selection tool to select the undesired area and remove it with a hit onto the delete key on your keyboard.

greetings K-Duke