Currently I maintain all "mass icons" (browsers, os, flags, ...) in my repository https://github.com/Findus23/device-icons and create pull requests against the main piwik repository on changes.
I want to change this to directly using the repository as

• it makes maintaining easier as someone who wants to add a icon only need to create a pull request agianst the repository
• it doesn't bloat the main repository as binary files don't diff nicely

While submodules would be easier for testing, it seems like they are only used for optional plugins in piwik. And it would add the whole repository (not just the resized images), but this could be solved in the build script.
On the other hand, it's possible to exclude all unneeded files from github zips (see https://github.com/Findus23/device-icons/archive/1.1.0.zip), which in theory should also exclude them from composer packages. (I haven't tested it)

What would you prefer?
(PS: In both cases it might be useful to move it to the piwik organisation)