Usually serving images from a merchant's server is quite fast... sometimes faster than your own server. Browsers handle caching and many merchants use CDNs to deliver images faster.
Most likely the reason that %li and %lt weren't working is because your URL and Path fields under Product Images and Product Thumbnails are probably not configured properly here: WordPress Admin Area > Tools > Datafeedr > Directories.
I would double check those paths to make sure that they are correct and adjust them if they are not.
You need to make sure your URL fields point to your correct image and thumbnail paths like this:
You'll also need to make sure your Path fields point to your correct image and thumbnail paths:
If that still doesn't work, it might be an issue of the merchant imposing image access controls preventing them from being downloaded to an external server.