Right now, there is no delete option from the Edge Library Panel. You have to right click the images, click the "Show in explorer" that will take you to the images folder, and then you have to manually delete them from the folder.
But the next release, you may find such option so that you can delete the assets from Edge Animate only. Edge may also have a clean-up button that will delete the unused assets from the composition.
I've been deleting images from the Finder (I'm on a Mac) but then when I save the Animate file I get an error saying the image can't be found. Every time I click "Save". I check the <project name>_edge.js file and the images I removed in the finder are still there. Is there a way to force Animate to rebuild or clean up that file? I'm guessing I get the error because the assets are still listed in that file even though they are not in the images folder.