I doubt you would be able to find the sprites/assets without directly going into the game files themselves as BtD isn't open-source. That stuff kinda leans towards intellectual property, which I doubt David would publicly share.Is there a gallery with all the sprites and assets somewhere?
That being said,
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(526 images inc. in-game scenes), though it's only updated to mid-January of last year. Dunno if anyone else has a complete one of just BtD elsewhere.