How do you make your decal on roblox invisible on everywhere but the picture

first go get gimp 'cause I don't think you'll find an other free editor as well gimp.after you have gimp push open then DRAG the file into the empty space now along the top click on colors go down to color to alpha click on it a preview will pop up it will be on white to start this means white will be invisible you can change to whatever color you want after you're done press ok save go to roblox.com or click on play roblox or roblox studio go to my roblox stuff decals create browse find your picture upload it and tu-duh!
-myrobloxman