image size
Upload images at 1500 x 600. This size is ideal for the cover, and Notion will automatically scale them down for grid view. The high resolution ensures good quality in both displays.
However, for content to be properly displayed in both full-size (1500 x 600) and grid view (256 x 133), center the main content in a box approximately 750 x 390 pixels in the middle of the image. This ensures visibility in both views.
image background
background should be white or black or colorful, or at least not transparent, since this may clash with “dark mode”