Here at Panda Studios we made a thumbnail creator for our games to speed up the process of creating thumbnails of our assets to use inside Slate and Widgets. The original process was a very long and an inefficient approach to creating simple images. Because we do value our plugin we have released it to the public, free of charge, to use in their projects to allow you to create quick thumbnails of all your assets.
This plugin is made with our own needs in mind, and is therefore designed to fit our requirements. However, it should be sufficiently effective for projects with likewise goals. It automatically exports to .png and saved as Texture2D inside the content browser.
You can find our plugin on Github (GitHub - CelPlays/UnrealThumbnailGenerator), where you can either download source or precompiled binaries for 4.18 and 4.19 and fine more info on how to use this plugin.