I dived sort of deep into the resources of Roblox and noticed something on that what they refer to as the ContentProvider.
So I enabled this Flag FLogContentProviderRequests to 6. This Fast Flag will log this part of system into my Studio Log file now.
One of the things it logged was right after requesting the Decal.
[FLog::ContentProviderRequests] Parsing content: rbxassetid://Decal_ID
Then it came back with the Image ID.
So, it seems like that the Roblox Engine has a built-in for it but only in the C++ Level, it seems like. And it doesn’t seem to be available as a Luau scriptable method. I don’t believe that I found a Security Protected method that would do it either, other than :GetObjects and the rest you’ve mentioned.
But I didn’t find a built-in scriptable method that would return the actual Image ID directly.
It looks like that Roblox has an XML Serializer, that’s what the ContentProvider seems to have been doing when it said “Parsing content”.