I’ve just read PCGamer interview: There is section there:

“Rich McCormick: When you release a game you tend to package it in with additional materials. How do you track them down?
Guillaume Rambourg: It’s a lot of digging, a lot lot lot of digging. The thing is we have a design team on board and they are extremely creative and productive. Plus, our testing team, they still have many, many copies of old games in their attic at home. They are collecting games, so it is a lot of hunting, digging, and then a lot of work.”

An idea came to my mind: can community offer you some help with this digging? There are LOTS of retro gamers on gog.com – they have in their possession LOTS of boxed versions of good old games and I’m sure they will be more than happy to share. So if gog.com is going to release a game and has in it's possession : game itself, manual and soundtrack – but doesn’t have high-quality scans of boxes from various editions (US, EU), scans of floppies, &CDs etc - just specify technicalities and let us know what you need.

I'd be willing to rescan some of the manuals that have ReplacementDocs watermarks on 'em. :p

Well, when I say “extras” I don’t mean “home made” avatars, wallpapers etc. I’m rather thinking about official printed materials present in boxes. I especially miss scans of boxes & floppies & CD’s from various releases. It would be nice to see how did Fallout box and it's content looked when it was first released in US back in 1997 and how it differed to EU release.

And to the process, just few ideas:
Fist stage is to organize extras into categories (box, floppy, cd, soundtrack, avatars, manual & reference cards etc) with some descriptive fields – to have coherent collection of extras categories for all games.
Second stage is to specify naming conventions, acceptable resolutions and file formats, color palette, picture size etc.
Third stage is to offer volunteer job to some people from community as “game editors”. So every user will be able to send missing extras to the game editor and he/she, after confirming that received material complies with all requirements - will be able to upload it to the extras section of specified game (or send it to gog.com for upload)
I realize that entire process would be time-consuming but it will be easier than digging internet - and can offer better results.

Problem would be with publishers' rights I think - but correct me if I'm wrong.

I was just thinking of the same thing, please let us help, GOG :)We don't really know what kind of contracts GOG makes with the publishers, so can't correct you... I would hope they get the right to redistribute certain extra materials for all the games.

^This.

For cover art go to MobyGames.

I could help collecting with certain games. For example do I have countless of screens,wallpapers and avas(screenserver and webkits) for the Prince of Persia series.

Naturally, they can't use anything without the rights of the publisher. But I think in many cases, especially with old IPs that have changed hands a number of times over the years, the reason some extra or other is missing is not that the publisher won't allow it, but simply that they don't have it.

I imagine that often when a company goes bust, the assets are sold off in various chunks, and lots of materials are probably lost. Another company may buy the IPs, but not necessarily get their hands on all the original artwork, music, etc., even though they now hold the rights to it.