How your map sits in the viewer comes down to two things: the shape of your image, and one setting that decides whether the map fills the frame or sits neatly inside it.
The shape comes from your image
Mapvera works out the frame from your image's aspect ratio, its width compared with its height. A wide image gets a short, wide frame; a tall one gets a taller frame. So the single biggest lever is how you crop and export your artwork. For the full picture, see Aspect ratio: how your map fills the screen.
The Fill the container setting
On the Settings tab, in the Zoom section, is Fill the container ("Keep the map image covering the whole container at minimum zoom"). It's a desktop setting and is always on for mobile.
On. At the most zoomed-out point the image still covers the whole frame, so there are no empty bands around it. Some of the edges may be cropped from view until the visitor pans.
Off. The whole image is always visible, which can leave plain background bands where the image shape doesn't match the frame shape. Those bands show your background colour.
Tidy the edges
If leaving Fill the container off shows bands you'd rather not see, set a background colour that suits your artwork so those areas look deliberate. See Set your map background.