I have been doing glass ornaments like these for a couple of years now, I love to get new stamps every year. The next 4 are snow globe style and they are tipped upside down into a decorated (clean) cat food can, you must hot glue it into place.
The images are stamped with Stazon (jet black) onto transparency papers then colored in with acrylic paint. Cut the image out so it fits into the ball (I recommend making a template to trace) then wait till it dries roll it up push it in and then it pops open. I would also leave a small tab at the top so that is stays upright and doesn't roll around too much!
How cute is this House-Mouse stamp.
Images are inside the ornament if you can not tell from the picture.
When I first started doing these I colored them in with sharpies and used a glitter that stuck all over the glass. They were cute but the image wasn't real clear and you wanted to try not to shake them. Then I used paint pens to color and chunkier glitter, better but still not as clear!