That picture is absolutely possible to describe. Imagine that someone says to you, "I can't see that picture; what is it?" Would you really tell them "I don't know how to put it into words, it's completely impossible to describe"? No, you'd tell them what it is. So do that.
In fact, the picture contains a description of itself, so you can just use that. You can describe this picture as
A comparison of the median estimates of the CSCC for the four BHM damage model specifications.
Or, perhaps, you could say it's
A picture consisting of four world maps, each colored according to one of the four damage model specifications.
Even "Some maps with text underneath" would be better than no description at all.
Even if a picture doesn't contain any recognizable objects, you can always describe it somehow, as in "A colorful picture with a variety of shapes, but no recognizable objects" or "A blurry picture with subdued colors and a handful of unrecognizable shapes."
There's no such thing as "an image that's not really possible to describe."