After Alessio explained to me that the cover quailty is from the site the script is pulling from and not the program I tried DVDEMPIRE and got very good covers.
The cover's quality depends on scripts (and then to webiste where you get informations). For example IMDB has a lot of covers, but low quality, you can try specific script for covers.
I found out a while back about the quality of IMDB covers (NOT a problem with XMM).
What I do is set my script configuration to get all information i want from IMBD except the posters and covers.
Then I change the script configuration to get the covers from places like Amazon.com most of the time (I get only the covers, not the posters) or Whoever has the best quality covers.