I only use screenshots with mamewah and do not use any boxart. So I am not willing to make a boxart package. However, I would be happy to host one.
atariage has excellent cartridge scans and box scans for about 3/4 of the games I had screenshots for. Here is the process I went through to collect all of the screenshots. It is rather tedious.
1. I had 2 windows open and right next to eachother. They were my good named roms and the website.
2. I would do a single click on the rom file name and then copy it.
3. Then I would goto the corresponding screenshot rightclick on it and do a save as.
4. Next I pasted the rom filename for the picture name.
5. Repeat 1-4 490 times

I did not resize any of the screens and I left them in the original PNG format.