One of the most common issues artists run into in the distribution process is 'Blurry artwork'.
The streaming platforms have strict rules on artwork, and they can (and will) reject your whole release if they aren't happy with the image quality.
You don't want blurry artwork to be the reason you can't get your music out there, so make sure you're starting with a high-resolution original image! You can't edit an image to be more high resolution, so it's crucial that the image you begin with is high-res.
Do the corners and lines in the image look smooth?
If you open the image fullscreen on a monitor, can you zoom in slightly without seeing individual squares of solid colour (pixels)?
Above are two same-sized images. The one on the right is too blurry!
If you look closely at the top of Anthony's head, you can actually see big blocks of color where there should be smooth lines.
Using text on your cover art
In addition, the streaming platforms are super strict on the legibility of text on the artwork, so you have to make sure your font choice, font size and font color all make any text easy to read, even when shrunk down quite small.