Why are my pictures being blurred

I have my pictures measured correctly. It would insert the picture using the add media button (upper left). It defaults a size to 300X82. I need it at 560X150 which is the original size the banner was made at. So I typed in the size. When I go to view it, its slightly blurry. So I made new larger re-sized banners and tried again. It seems to blur my banner no matter what. It will not give me the size option I want the banner at without blurring my banner. What do I do?