The rules that get releases rejected
Stores reject artwork for a short and very consistent list of reasons: it is not exactly square; it is smaller than the minimum; it was exported in greyscale or CMYK rather than RGB; it carries a web address, a QR code, a social handle or an email; it shows a streaming or social media logo; it advertises a price or says "Out Now"; it mentions a physical format like CD or vinyl; or the text printed on it does not match the metadata submitted with it, character for character. Each one of those is a fixable mistake found in seconds and discovered, usually, three days before release.
Size, and why 3000 pixels
1400 × 1400 pixels is the published minimum on the strictest major service. 3000 × 3000 is the recommended size and the one used for the largest displays, including the ones your listeners see when they cast to a television. Export at 3000 from your original artwork. Do not take a 1000-pixel file and scale it up: the upscale is visible, and it is a rejection reason on its own. This tool estimates fine detail so you can catch an upscale before a reviewer does.
Colour mode is the silent killer
Design tools that default to print output export in CMYK. Photo tools sometimes export greyscale when the design happens to be black and white. Stores require RGB, and specifically sRGB. A CMYK file often looks fine on your screen and is rejected on delivery, which is why this catches so many people. If your artwork is monochrome, it still has to be exported as an RGB file.
Design for 36 pixels, not for your monitor
Almost nobody sees your cover at full size. They see it in a playlist row, in a search result, on a lock screen and in a mini player. That is between 36 and 160 pixels across. Fine typography vanishes, thin strokes disappear, and a busy image turns into noise. The preview on this page renders your artwork at those exact sizes. If you cannot tell what it is at the smallest one, the artwork is not finished.
What this tool cannot see
Software can check geometry, colour and detail. It cannot read your artwork the way a human reviewer does, and it cannot know what your metadata says. Whether there is a logo in the corner, whether the text matches the title you submitted, and whether you actually hold the rights to the photograph and the font are things only you can confirm. The checklist under the results covers those, and it is worth reading before every single release rather than once.