Here are all my variants and attempts on uploads
This one is white so you can't see it unless the background is set dark https://www.roblox.com/library/1898679429/Aimthinger
https://www.roblox.com/library/1872093750/mOUSE
https://www.roblox.com/library/1852771339/sight
mouse.Icon = "http://www.roblox.com/asset?id=18986794329"
I heard that you're either suppose to change the ID to one less or one more i forgot, but I tried both and neither work. On other decals when I make the ID's one less, it goes to the same decal,but somewhere else. When I do it to my decals it takes me to a whole to item uploaded by someone else.
Upload them as a decal to roblox. Apply them to a part. Go to properties. Roblox studio should have formatted it correctly for you then.
The problem is that the decal ID (what you see on the website) and the actual texture ID (what gets loaded) are different. Place your decal anywhere and copy and paste the asset under its "Texture" property into your scripts. This will be the correct ID.