With boxart makes this a bit more tricky, even more so with downloadable games being a thing for many devices that fall under the viable emulation umbrella, never mind hacked system umbrella. Offline list and anything contemporary with it is probably still going to be where you start here if covers is a thing.
As far as dat files then
https://no-intro.org/ http://www.advanscene.com/ and whatever remains of TOSEC
https://www.tosecdev.org/ probably ought to be mentioned. MAME/MESS kind of does its own thing. Some might even still suggest goodtools
https://emulation.gametechwiki.com/index.php/GoodTools and there is an argument to that as most of the previous stuff cares more about purity, or maybe Scene releases, and if you can get goodmerge
http://goodmerge.sourceforge.net/About.php going on (one of the few things to try merging ROMS to get storage down even further -- compressing a ROM is good, compressing the three basically identical ROMS that form region dupes is better for pure storage concerns) then there is something there.
Most boxart is going to be for fancy loaders on the device itself -- the Wii had endless fun with covers (see coverflow for but a start into that world) here, the 360 has its own stuff too for various things (be it ODDE* or homebrew capable efforts).
*optical drive device emulator. You basically fake the DVD drive with an adapter and fire across region compatible ISOs from a USB drive, they often featured secondary virtual USB things to allow you to use the onboard picture viewer - go view the boxart on this virtual USB and as it knows what you looked at it loads the corresponding iso into the ODDE aspect. Ultimately a rare device (far cheaper to flash the drive and burn dual layer discs, and not as nice as a full homebrew setup that dodges region locks, allows DLC, XBLA, homebrew, mods, USB, ping limit removal and more besides).
Most consoles also then got specialist ROM management setups on a per console basis if they were doing nice UI as opposed to simple listers like most people use the no-intro stuff for, some of which do cover boxart/screenshots/title shots. It gets even more complicated on the Wii as you also get to contemplate what you are doing here as far as unscrubbed, scrubbed, super scrubbed and WBFS based affairs, even more so as there are now tools to go between all of them if you so desired.
Basic scrubbed vs unscrubbed is probably the big one -- Wii games padded out their isos with data that no piece of anti piracy, stock device, emulator, ROM hack (technically), flashed drive or similar has ever cared about or will ever care about (give or take a hypothetical anti piracy emulator in the future) as it is literally random junk. Removing it then allows you to go from full DVD size iso to allowing you to compress it down to whatever size the actual (encrypted) data is and that can be huge -- New Super Mario Brothers is a bit of an extreme example but goes from 4. whatever gigs to a few hundred megabytes. Some take it a step further and go for full WBFS as their storage medium of choice. None of this is great if you are going for traditional ROM management setups that just want to do a CRC32/MD5/SHA1 of your ROM/ISO (maybe extracting it from a zip/rar/7z if it is good) and say yay or nay.
