I'm not sure of the format of the images from GD3, but when saved to the folder they are jpeg. Most images are 1000x1000, which look nice on my TV mediaserver UI. My CD collection is a small sample size, but when compared to musicbrainz and freedb, GD3 seems to have more consistent results meaning less manual editing.
I'm not a programmer, so it's not of much use to me, but there's an api posted at the following link, maybe someone could take a look at it:
http://www.getdigitaldata.com/GD3API.aspx