I'm glad you posted this. It's the first of your work where I noticed your name and resolved to seek out other work by you, as I liked it so much. Mostly, however, I'm glad you posted it for the chance to raise what may be a sensitive question, regarding a problem I see with much fantasy illustration, which tends to involve the depiction of a pre-modern, generally medieval milieu. The characters look more like modern people in dress-up than actual people of the period.
I can't quite frame what there is about these depictions that suggests moderns in dress-up to me, except to say by comparison the figures of Jon Howe and Jon Rush look "period" where most other fantasy illustrator's figures don't, yours included.
But is it even a problem?
This work is a cover for a book about a teen-age girl in Arthur's day. The target audience, presumably modern teen-age girls, will relate to an image reflecting their experience, that's just good-sense marketing. This all assumes the anachronism is intentional, rather than an accidental by-product of using photo-reference, of painting in fact "a modern person in dress-up".
After all this, I guess that's the question, is the anachronism intentional or accidental?