Owl of Oban is quite right, people would dress up in the imagined attire of the
'wild untamed highlander', and so be pictured.

When in fact they'd probably wear trews or even the normal attire of the English upper classes: of which they saw themselves as a part.

The point being that the kilt is practical wear-imagine wearing a fancy hairy sporran on the hill all day, and in all weathers?

To get the point, look at the miltary portraits of the second part of the 19thC-all the finery: and then look at actual photographs of the time.

James