actually, a lot of GFX-ers use it to create depth and more flow.
AND, A C4D is a Computer-Generated Render (it is made in Cinema 4D). Like I said, A lot of GFX-ers use it to create Depth and add more flow. It is also used as effects, such as Lighting's and such. You can use it for Vector, smudge(it can be used to create more smudg-y effects), and the like.
if you want to get started, try downloading some Packs In Deviant make sure it is in ZIP file)
AND a stock is a picture of any backgrounds(Like mountains, road, the likes) or even pictures(Renders, C4D's, Fractals, ETC). it is used to create scenery, Texture, and some other stuff.
AND yes, save your files In PNG
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