Add missing pictures from device pictures source

[maurerle]

When using the source from here:
https://github.com/freifunk/device-pictures
many additional pictures have not yet made it to this repository.
One solution is of course to use the device-pictures repo as a source - see #154

But as others might not want to use this, I want to mention that these pictures are currently missing in this repo:

• COVR X1860
• NETGEAR EX3700
• Ubiquiti⁣ EdgeRouter X
• Ubiquiti⁣ EdgeRouter X-SFP⁣
• Xiaomi⁣ Mi Router 4A/4C (multiple editions)

As I do not know the exact command used here to create the 256x256 jpgs - I am not filing a PR for this.

[herbetom]

Using those pictures is currently kinda blocked by #158.