Make sure the ‘show asset properties’ mod is installed.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=451710073&searchtext=

Open an asset that has a thumbnail, open properties and click ‘copy’. (nothing seems to happen but is ok)
Open an asset without a thumbnail, open properties and click paste. (nothing seems to happen but is ok)
Click ‘tools’ icon to make a screenshot. With camera mouse click on object (nothing seems to happen but is ok)
Press ‘print screen’ button on your keyboard and paste the screenshot in an image editor
Click menu button (right top) and ‘Save Asset’

KEEP THE SAVE SCREEN OPEN and go to
c:\Users\YOURNAME\AppData\Local\Colossal Order\Cities_Skylines\AssetStagingArea\
There is 1 temporary folder with images of the current asset!

With your image editor, crop the screenshot and save it as thumbnail: asset_thumb.png
Save a larger version as asset_tooltip.png
As soon as you save the image, it will show in the save screen!

Sizes:
Thumb: 109x100
Tooltip: 492x147
Snapshot: 644x360

You can find the snapshots at
c:\Users\YOURNAME\AppData\Local\Colossal Order\Cities_Skylines\Snapshots\

Now save the asset, and that’s all!

To get to the temporary images folder there sometimes is a little folder button in the right corner of the images, but it is not always there. Anyway - there is only one folder in \AssetStagingArea\.

I fixed all my missing icons that way, and have them locally in the Cities_Skylines\Addons\Assets\ folder.

Unfortunately i can't upload them again without some angry faces from the original authors.

Hope this helps!