You only have 720X480 pixels. Where do you want the quality to come from. I suggest you export and then set Photoshop attributes to 300 dpi. Much better, right? But smaller. Too bad. That's the breaks.
Seriously, there used to be a product on the market to get better frame grabs by estimating the pixels when creating a denser picture. It worked with the old Premiere and maybe with older versions of Premiere Pro, but I don't think it got updated. Maybe InstantHD or ReSizer would help.