The standard corner rounding that is applied to card images in decks and other areas of the site is not applied to images in primers. When applied, this style makes the cards appear more natural and hides the white corners added to most source card images. Without it, cards stand out as having artificial white backgrounds, especially in dark mode.
Either apply that style to all card images in primers or enable authors to apply it themselves through markdown.
Risks: applying the style to all img tags in primers could make non-card images appear in unintended ways. There should be some control within the img tags or some way of specifically inserting a card image that would inherit the style.