Getting rid of the background colour is possible in photoshop or many other photo image tools using the magic wand to select background and then deleting it. Making it smaller is a matter of making the image the correct size. I usually create a transparent canvas just over the size in mm I want the image then drag the artwork onto that and resize. At that point it becomes so tiny you need to show the image at much greater than life size to work on it(especially when decals are for smaller models or tiny items... ).
It will often print well so small you cannot read it, but when photographed and magnified it is pretty accurate. That said a reasonable quality ink jet printer with multiple individual colour cartridges is best and print at photo quality onto decal paper. I use a Canon MG5750 for example, other makes are available!
But after all that, silver will be a multitoned grey, and gold a dirty yellow. And when placed over some base colours the decal simply doesn't show up.
Trial and error seems to be the only way with successes sometimes and failures at others.