You must upload your cover as a jpg. file. One of the sides must be at least 3000 pixels, but no more than 5000.
In addition to the technical requirements for the cover, there are also the qualitative requirements of the stores. The cover must not be pixelated or blurry when you open it in 100% size, and only the title and subtitle, if available, as well as the author may be mentioned on the cover. In addition, the publisher and the series may be added - unfortunately, no other additions are permitted.
Further details on the quality requirements for the cover can be found here