Seal size

[tdyku]

For now seal size depends on image height:

main = Image.open(main_fname)
main_w, main_h = main.size
...
ratio = 0.15
logo_h_new = int(ratio * main_h)

It's good-enough heuristics for square-like images, but makes seal too big when working with high and thin images (posters, UML diagrams, etc.)
It would be good if we could customize size depending on width or with fixed size.