How to remove the quality on my background picture on the APPLE LAPTOP D;?

i just added pictures to my macbook to change my backgrund when ever i put a picture to the background pic i came out to be crappy as like in the quality of the pictuer shown in the backgrond i looked all over hte internet and i couldnt find the answer to it i only saw how to remove the quality on on the macbook ... soo how do i remove the quality on my background picutre.... OR dose it depends on the pictuer itself? Thanks~