[Feature Request] Option to add custom images for height comparison

[mechobree]

It would be very useful to be able to attach custom images for size comparison, for uses such as seeing at a glance how our own ocs look next to each other height wise.

[snowsune]

I like this idea! I might have to expand the rendering system a touch, because it expects data for any species you load in. So might be a good idea to have a blank/no-species option, that way you can upload your character (you'll just have to type in exactly the height you want/expect without the calculator doing it for you)