How to center rubber image with flexbox caption?

Question

css

There is a layout with a picture and a caption, both can be of any size. You need to center them.

The layout should behave like this (refer to the picture above):

1. If the picture and signature fit into the screen, then nothing much happens. Everything is in the center.
2. If they do not fit in height, the picture shrinks until everything fits.
3. If the picture does not fit in width, it shrinks until it fits.

How to achieve this behavior with flexbox?