I'm pulling together the finishing touches to teach Scratch 2.0 and App Inventor later this month and figured out a cool way to access the Scratch 2.0 image library, including custom sprites you make using Scratch's Paint Editor. Remember, Scratch 2.0 is totally cloud based, unlike Scratch 1.4, where you installed the entire sprite library onto your hard drive. The good news is that 2.0 allows for a lot more collaboration between users. However, it does make exporting sprites for use in other applications a little less intuitive. Sprites from the 1.4 library live in your 86x program files, so easy enough to locate them and upload to other platforms. In 2.0, when you export a sprite (R click>save to local file). you'll get a file with a .sprite2 extension. The Paint Editor wiki reassures you in the "Interaction with Other Programs" section from the link above that you can create png and jpg files from bitmap created custom sprites (this is the default type Scratch takes you to in the Paint Editor; check out the full wiki entry for a very good, but detailed description about bitmap vs vector image properties). Well, there's a bit more to it than that, but close! Once you've saved your .sprite2 exported image, rename it, changing the .sprite2 extension to .zip. For example, changing a file named "Sprite1.sprite2"  to "Sprite1.zip". Once you open this zip file, you'll see at least 3 files within it, one a png file. Extract this file and now, you can upload it. Easy enough.

I prefer using this approach to creating custom sprites instead of using external editors such as GIMP or Paint (both very nice freeware tools), because Scratch's Paint Editor creates images that fit very nicely within the App Inventor screen dimensions; there's very little pixel manipulation needed to get the sprite you create to work within a project. And I learned this past summer that kids love Scratch because it gives them so much artistic license, so be prepared to encourage this!