Correct, images aren't part of a symbol definition, as symbols usually are simplified, functional, descriptive drawings of real devices, sometimes even broken up into several sub symbols.
Why don't you put this image (scaled correctly) behind a see-through symbol if it's a one-off event?
scaled up (factor 200):
Color values might not all be exact the same as I just scaled the image from above down to single pixels with weighted average function (grey patch left top is a little bit lighter than the one in the top center for example, i.e. RGB values of patch x are not exactly RGB values of patch y, although they look the same).