I assume you are using a HFS+ formatted USB device?
If yes, there is a bug confirmed by the development team based on a ticket I raised back in January. The CDJ-3000 reads the 'wrong' AlbumArt from the USB device. There are 2 AlbumArt stored on the device for the same track. A small one (80x80 pixels) and a larger one (240x240 pixels). In the Jog display it should show the larger one which would be very clear. But if it reads the smaller one it will look blurry.
You can test it by manually renaming the 2 pictures on the device by exchanging the name. Once done the picture will be shown correctly.
If you format your USB with FAT32 this doesn't happen.
The development team confirmed in my ticket that the issue will be addressed in a future update which might come in a couple of weeks.
If you are using a FAT32 formatted drive and do have the issues, it might be a different issue.
The issue is with HFS+ formatted devices only. In Export Link mode or with FAT32 devices it works normal.
Hope that helps.