Raster format is also known as bitmap or BMP. They mean the same thing and are comprised of pixels, so when you enlarge them they become diluted. It's the most widely used clipart format and is usually all you need.
The type of clipart that I use is important for all of the different effects that I'm creating. It needs to be a vector drawing for sure, as non-vector drawings are bitmaps that don't resize nicely... they'll just get all bumpy and not look good. Also, I'm drawn to simply line art because if it's too complex sometimes it doesn't look as good in the different sizes. It's also good if it's a single color, usually black, because then I can change it to whatever I want quite easily just using simple graphics software. Just remember, simple is better, is cleaner, and usually creates a more sophisicated look.
Various websites contain thousands of readily available images. These drawings are further grouped under sub-categories; say baby girl images, baby boy images, baby shower images, etc. One can avail the services and select a single drawing or a group of drawings from a particular sub-group according to ones needs.