This isn't so much of a bug as it is a lack of consistency in default settings, which can be confusing to a store webmaster.

The image used as the category icon generally has a dimension ratio of approximately 1:75 width to 1 height. This setting is controlled in Admin Config Images in three places:

Subcat Image Width: 100
Subcat Image Height: 57
(this controls dimensions of category icon images, including top-level category icons if home page is set to display all top-level categories, when you are viewing a page with a collection of categories.)

Top Subcategory Image Width: 150
Top Subcategory Image Height: 85
(this controls the dimensions of the top-level category icon image, when the top level has subs and you are viewing the page showing the collection of subs)

--- however ---

The following setting for the category icon image, which controls dimensions of the category icon on Product Info pages (and on a completely empty category page), has default settings for dimension that are not consistent with the above default dimension settings.

Category Icon Image Width - Product Info Pages: 57
Category Icon Image Height - Product Info Pages: 40

Image Handler conveniently resizes for category icon images so, bottom line, if you have an icon with native dimensions of 150x85, then w/ the default settings above it is going display on the Product Info page (or empty category page) as 57x32, not as the implied 57x40, so this could be confusing to some people who haven't compared these three settings for the various category icon image areas of Zen Cart.