@PeverelPointI started out as an artist, and have always designed my own book covers. Now I mostly use stock art, rather than 'freeware' art, but my history is design helps with that as well. Unfortunately, both art and writing are, at least for me, a full-time process, as if you only do it occasionally, you don't develop and maintain the same skills. Thus the knowledge is there, yet I can no longer draw a straight line to save my live.
I have NEVER tried to illustrate characters, as the whole point of a cover ISN'T to actually depict a specific scene from the story, instead it's designed to get readers interested enough in the overall story conflict to PURCHASE the book, and that's always been my guide.
Besides, readers prefer replacing characters with their own characteristics. If there's a certain feature I want to focus on, then I'll continual reemphasize it, knowing the readers will reimagine them. So given that reader tendency, why bother? It's a non-productive extravagance, of only limited value.
Physical body characteristics are essential (ex: thin, fat, large and hulking or sleek and graceful), so too are small breasted, large breasted or ginormously breasted (I find those more wish-fulfillment than realistic, as most women with breasts that size often struggle with it, so they're often to be more disinclined to do other things).
In other words, focus more on body types, rather than focusing on individual features, because again, readers tend to superimpose their own preferences, yet body types determine what the characters are capable of.
Yet, on SOL, such advice is rarely listened to, as each author tends to have their own preferences/kinks too. Everyone in my body has ALWAYS been thin and athletic, and even the black women I know are also thin and athletic, mostly track stars, where they develop those secondary sexual characteristics much later than other, more sedentary girls/women.
Thus, the figure on the cover has NOTHING to do with the character in the story, and however you 'paint' the character will last a couple pages to a chapter, at most!
And, stories are painted with words, so STOP thinking in terms of movie images, picturing movie star good looks. So describe how your characters respond and act, rather than how YOU perceive them.