iTunes has the ability to attempt to download missing artwork for audio files. Unfortunately you cannot add DFF, DSF or FLAC files to your iTunes library. Further, while iTunes can contain WAV files in its library it will not process artwork for WAV files.
Using the batch processor and the Create Stub and Process Stub action functions there is a workflow you can use to have iTunes attempt to find missing artwork regardless of the audio file type. More information can be found here.