importing block and small pencil issue

hello,

I have been trying to import block images which works fine for some blocks like the diamond block. i am able to import it and layer it and add the colours i need properly.However, i tried doing the same with emerald or just the diamond drop using a png file and the importing works fine but when it comes to drawing and changing the colours and layering it the pencil is very small and changing its opacity doesnt help. unlike the diamond i can just change the colour by clicking and is directly where the squares should be. Is this becase imimporting bad images? thank you.