That is correct. Passing Alfred a URL for an icon doesn't work. You have to cache the icons locally and pass Alfred the filepath.
That's exactly how you have to do it.
I wrote a module that might help. It generates thumbnail images, but it could easily be repurposed to download images instead. The general idea is that if an image isn't already cached, it queues it up to be downloaded in the background, so your workflow isn't slowed down by waiting for images.
This. Foot-guns are a big no-no. Features are generally measured against what would happen if they were used with reckless abandon rather than with sound judgement and great care.
