Just thought I'd contribute back a lttle, so I made a pack of covers for games I have, and those I aim to get at some point (Mostly PAL, but some NTSC and a couple translated NTSC-J Games). Obviously, if you have a game that has the cover art here, but of a different region, simply change the TitleID in the image title to reflect the version you own.
Each image is named correctly (TitleID_COV.png) and has a standard size of 140x200 (As suggested), although, the odd few are a few pixels off, either width or height, so as to keep the correct ratio of the original cover image I converted it from (Please note that these will still work fine, and will still look the same size as all the rest.)
Instructions : The attaced zip file is called ART, so you can either exract the contents to your own ART folder, or extract the zip as a Directory to your OPL games folder. Thats it.
(NOTE: I zipped these with 7zip, and although I am pretty sure most archive software will extract it ok (Winzip, winrar, jetpak, etc..), if you do have problems then just use the latest version of 7zip, its only a small download.)
I hope these can be of use to somebody.