Normally you just have to subtract 1 from the DecalId a couple times to get the ImageId of decals you upload. I just had to subtract 1 seventeen times from this decal to get this image (same with this decal and this image). I imagine this was an oddity, but I suspect the distance between decal IDs and image IDs will only grow as ROBLOX gains more developers and people are uploading more assets. It’d be great if we could copy the image ID right from the decal’s page in the gear dropdown. Mockup:
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What we really need is for ContentId parameters to accept a Decal asset where an Image asset is expected. Then you’ll never have to think about the fact that there’s a backend Image asset attached to your decal.
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But ‘what we really need’ do you mean ‘soon’ ? 
No<!–>
Awww
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