Pixelation is when individual pixels can be seen when viewing an image at its native resolution.
The image above looks BAD because it is pixelated.
The image above looks GOOD because it is not pixelated.
What causes Pixelation?
Pixelation is normally caused when a user attempts to create an image to meet the minimum dimension requirements for cover art as set out by the music stores, but the dimensions of the source image the user is using is smaller than the dimensions required. This is known as "upconverting". When you attempt to upconvert an image, pixelation will occur because you can NEVER increase an image's dimensions without introducing artifacts like pixelation.
Why can't I just use any image I want for my cover art even if it is pixelated?
Because we are delivering your music into the top digital music stores. These music stores have specific requirements for cover art and intend to offer their customers with the best possible shopping environment and they don't want their beautifully designed store fronts to be cluttered with unprofessional looking cover art. To be plain and simple, pixelated images are ugly and the music stores won't accept it.
I don't understand, I converted my cover art image file to the required specs you stated on your site but it's still pixelated.
Just because you change your image file size to the specs required, that doesn't mean it won't be pixelated. You could take a 600x600 image file and upconvert it to 1500x1500 and it could still come out pixelated because the source was low resolution to begin with. Basically, if the original cover art file is low resolution, and you upconvert it to meet the minimum specs, it will come out pixelated and be rejected and you'll have to start over from scratch to create your cover art.
I can't do it. I'm not a graphic designer.
We all can't be perfect. But thankfully, that's why graphic designers exist. We would recommend hiring a graphic designer to create your cover art.