GeoDirectory doesn’t require fixed image sizes, but the two category images work differently, so they need different sizes.

Category map icon

The Map Icon you set when editing a category (Places > Categories) is shown on the map at its own pixel size. GeoDirectory reads the width and height of the file and uses them for the marker, so upload the icon at the size you want it to appear. If GeoDirectory can’t read the size it falls back to 36 x 45 pixels, which is a good size to aim for. Use a PNG with a transparent background so the map shows around the pin.

If you leave Map Icon empty, GeoDirectory generates a marker from the category’s Category Icon and Category Color.

Default listing image

The category’s Default Listing Image is shown for listings in that category that have no images of their own. There is also a site wide fallback at GeoDirectory > Settings > Design > Archives, Listing default image. These are used anywhere the GD > Post Images element shows listing images: archive cards, sliders and the header image on the listing page, depending on your theme.

Because of that, size them for the largest place they appear:

• Use a landscape image. GD > Post Images shows images in a 16:9 frame by default.
• If your theme shows a full width header image on listings, upload at least as wide as that area. Around 1400 pixels wide or more is a safe choice for full width headers on desktop screens.
• WordPress makes the smaller sizes for you, and the Image size option of GD > Post Images decides which size each element loads, so a large upload doesn’t slow down small cards.

More on this: GD > Post Images and How to resize GD post images.