Yuck! I think it's much better to teach w32-win.el and
w32_delayed_load to load more than one library for a certain image
type.

Note that although such modification would be reasonable, still

'(magickwand "libMagickWand-6.Q16-2.dll")

'(magickcore "libMagickCore-6.Q16-2.dll")

is consistent with what you already have for RSVG:

'(svg "librsvg-2-2.dll")

'(gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")

'(glib "libglib-2.0-0.dll")

'(gobject "libgobject-2.0-0.dll")