An image that contains semi-transparent areas that aren't part of the background. These areas will look different depending on whether the image is viewed in light mode or dark mode.
Typically found on Twitter, where images appear on a white background in the timeline or on post pages but have a black background when clicked on to show in photo mode. Japanese artists will typically indicate this with "タップ推奨" (suggesting to tap the image).
If it's just the background and it's fully transparent, use transparent background instead.
PNG and WebP are the most common image formats that support this kind of transparency.
