Some community members have created editor plugins that allow you to semi-automatically create icon images from item and building descriptors at a number of resolutions. This is helpful if you’ve already created a model for your content, and you want to create UI icons for it that are consistent with those from the base game.
Kyrium’s tool captures images using a render target, allowing capture of materials with opacity.
You can learn more about how to download and use the plugin on its Gitlab page.
Nog’s tool uses Unreal screenshots to produce images.
You can read more about how to install and use the plugin on its GitHub page.