Actually unless this series is ends up being SP, it's irrelevant.
If the Editor & I make these stories good and they get taken by a publisher, then I don't really have to worry about covers. Someone else's job?
do you think this pre-Raphaelite painting conveys the style, setting, and tone of your story?
The original doesn't 100% (no cats and Sorceress is only going to wear blue. I don't know why.) But oddly more appropriate than proto-Celtic Warrior with a big bronze sword
Stylistically my setting is mix of Elf, Dwarf, Dryad and Human cultures. The Human culture are at a mix of 16th to late 18th C. arts, styles and technology. But it is NOT the 18th C, it's now, but somewhere else. Apparently even in "real" Ireland in 17th & 18th C the latest Modes in clothes etc co-existed with a mix of previous centuries. People kept using what they liked.
The pre-Raphaelites had an artistic conception of a past Mediaeval age that was really fantasy I think.
I do think 4th Century warrior brandishing a sword would give the wrong impression. I think once some guards with pistols seem menacing and our merry band of Teenage heroes frighten some inept Sorceresses (descendants of Old Norse) before putting their knives away. There are no actual battles or swordplay at all. Nuada's Sword is carried (wrapped up) in case its vague magical properties might be useful. There is a similar design of Bronze age sword (made of Bronze) in the Museum in Dublin, but without the magic, some claim (with zero evidence) that it is his sword.
Many SF books have images of spaceships not actually in the story. At least I have seven female characters that are approximations to Sorceresses, Enchanters, Witches etc (1 elf, one dryad. Humans: one Tuath Dé, one part Tauth Dé, 3 x Vanir = Norse). The potion (an enchanted poison) and antidote are pretty central.
But it's only a thought. Perhaps my daughter will paint something (though she is somewhat impressionistic). I know two other artists. The Vanir (Norse) witches (Seidhr type magic) seem to have liked blue "work robes". There were three kinds; Volur, Seidhkonur and Vísendakona. Though I don't yet know exactly what those are yet. It's not critical as these people have been cut off from Sweden for over 250 years, I can make it up.