Category cover image

[mark@rs]

First time user - when I add an image (at the correct size) to the category cover image it only appears as thumbnail within the field.

Please advise how to make it work properly

[NemoPS]

Try to inspect the image in the front office, and see if it's the correct size (like right click and open the image in a new tab).
If it is, there must be some css constricting it. If not, it might be the category image setting in the bo.