The OP wrote artworks/concept arts so I'll take the risk of posting some of the Commodore 64 pictures I've done, they certainly aren't concept art but they might qualify as artworks. Some are in multicolour bitmap mode (160x200 pixels, 3 colours + screen colour for each 4x8 pixel block) and some are in singlecolour bitmap mode (320x200 pixels, 2 colours total for each 8x8 pixel block), a couple use sprite/raster timing trickery to put graphics in the borders and two of them use intense raster timing majic to squeeze more than 3 colours+background colour into each 4x8 block.
<img src="http://cheesepirate.com/c64/art/purple.png">
<img src="http://cheesepirate.com/c64/art/song_of_the_sunset.png">
<img src="http://cheesepirate.com/c64/art/funkscientist.png">
<img src="http://cheesepirate.com/c64/art/silence_of_the_moon.png">
<img src="http://cheesepirate.com/c64/art/sheep.png">
<img src="http://cheesepirate.com/c64/art/i_can_has_roasted_marshmallow.png">
<img src="http://cheesepirate.com/c64/art/weeee.png">
<img src="http://cheesepirate.com/c64/art/missing_you.png">
<img src="http://cheesepirate.com/c64/art/commoduck.png">
<img src="http://cheesepirate.com/c64/art/the_short_the_mule_and_the_ugly.png">
<img src="http://cheesepirate.com/c64/art/croc_&_possum.png">
<img src="http://cheesepirate.com/c64/art/aussie_animals.png">
<img src="http://cheesepirate.com/c64/art/sinners_all.png">
<img src="http://cheesepirate.com/c64/art/mermaid_vs_blackbeltjones.png">
<img src="http://cheesepirate.com/c64/art/muir_oigh.png">
<img src="http://cheesepirate.com/c64/art/arcane.png">
<img src="http://cheesepirate.com/c64/art/brute.png">
<img src="http://cheesepirate.com/c64/art/summer_in_the_city.png">
<img src="http://cheesepirate.com/c64/art/self_portrait_with_unicorns.png">