Hi Nick.
I got 2x CDJ-3000. The Artworks I am using are either 600x600 or 800x800 pixel. When exporting to a USB drive I got 2 Artworks per track with RB6. One is in 80x80 which I believe is because of compatibility to older hardware. i.e. my old XDJ-1000mk1 were using 80x80 Artwork.
The second Artwork on the USB device is 240x240. So I believe the CDJ-3000 or other newer hardware is using 240x240 Artwork.
When looking to them on the CDJ-3000 they are looking pretty good - don't show low quality or blocky. Are you sure your source Artwork is ok? If you have a low quality picture rendered to 600x600 or 3024x3024 and shrink it back to 240x240 it won't look good. Just in case...