Need quick help with graphics

orbnoob

so im making my pictures in pixlemator and im using 72 ppi, but all the pictures i make look so pixleated and tarrible, even a simple ball i make looks bad, i would show an example but i dont know how to post pictures here, any idea what im doing wrong? most of the stuff i make i use fill to fill an area, maybe that could be the problem, but i dnt know