Maximum size of images.
Emacs will not load an image into memory if its pixel width or
pixel height exceeds this limit.
If the value is an integer, it directly specifies the maximum
image height and width, measured in pixels. If it is a floating
point number, it specifies the maximum image height and width
as a ratio to the frame height and width. If the value is
non-numeric, there is no explicit limit on the size of images.