the icon in dialogs does not work like drawpic in @windows
you have to use the actual file width and height
to get them (from the help files)
$pic(filename)
Returns the size, width, and height of a bmp, jpg, or png file.
Properties: size, width, height
so use $pic(filename).width $pic(filename).height to get the image size
if you want to use an image but in a different size then you will have to edit it in a graphics software that can do what you want.
