This repository contains a large collection of game logos created for MAME as well as other platforms. I update it from time to time, adding new logos and fixing/improving others. In case you use the older packs that were distributed as .zip's, I'd recommend updating to the latest version.
In case you noticed a logo isn't present and care enough to let me know, I'd be happy to create it for you. Ideally you'd send me an unfiltered screenshot that contains the logo (typically the title screen), to save me from having to download the rom. I cannot afford to download terabytes of data I have negative interest in.
This one contains arcade flyers, game boxes et al, in the highest possible resolution and quality I could find, in .jpg format. I know there are comparable (probably more comprehensive) projects out there, but their content comes in .png format, which requires a (relatively) huge chunk of hard drive space and has no other advantage save maybe the frontend using it not needing jpg support.
Intended for use with crazyc's plugin that is distributed with MAME, these files rename the system's controls to something more descriptive (i.e. 'P1 Button 1' becomes 'P1 Fire'), which makes configuring them easier, especially helpful with fighting games with lots of buttons. A sizeable chunk of arcade games are done, but most home system definitions still need defining. I'm not likely to resume working on this, on my own, at least.