You can do some web scraping to get the content of any image with a decal ID on the Roblox website with roblox.com/library/DECAL_ID, but this method is inefficient and I’d rather use a documented API instead.
In addition, this page does provide asset details, but still doesn’t return a link to the image where I can download its content: https://catalog.roblox.com/docs#!/Catalog/post_v1_catalog_items_details.
How do I get the content of a Roblox decal from the asset library using an official, documented Roblox API?