Never put that you don't know what you want or "it's up to you". But you still don't know what should be on it? Let's fix that. In the publishing world, designers often already know what the book is about — or the publisher has briefed them. Here, we don't really have that. All the designer knows is what you tell them

i. Color
Color usually works in one of two ways: naturally, like how the sky is blue, or , like how blue can symbolize sadness or calmness. Sometimes these overlap: a calming blue ocean. Think about what colors can go with your story and how. Also remember that colors can symbolize to specific cultures.

i.e. A sad love story that takes place in the fall (and the fall part is important) 
Since it's a sad story, maybe blue would work, but if you want to go more for the autumn, maybe some orange/brown tones like dead leaves because love is dead red highlights are okay, but not too much since love and red are related

ii. Characters
Often times, characters are on the covers and you should probably tell your designer what they look like. They might not be able to get the best match, but at least they'll be in the ballpark. Also, what character do you want on the cover: the main character/narrator, a person they often see. If it's a mystery novel, what if its the person that's being discovered?Gender. Age. Race. Hair color/length. Eye color. What kind of clothes do they wear. Their aura.

Raven. 21. Caucasian. Long, semi wavy black hair parted down the middle. brown eyes. usually wears a cloak and a leotard. has a don't talk to me kinda vibe

iii. Mood
This is usually where people say dark, but be more specific. Do you want it to be spooky or edgy? Is it mysterious or action packed? Should I feel like something dangerous is about to happen?

iv. Content in relation to Imagery
Think about your title and then your images. If you're writing a werewolf book and you have "alpha" or "werewolf" in the title/subtitle maybe you don't need a wolf on the cover. There's no need to repeat yourself, this isn't a children's book. Maybe the images you show can speak to something else, what about your werewolf book is different ?

x. Symbols
Your symbols can be or literal. Apples are the forbidden fruit. The language of flowers believes that every flower can mean something, and the color of the flower can alter the meaning. But what about reoccurring imagery that you mention in your book? A family memento that keeps them strong. Something they're looking at every day. 

xi. Location
Where does your story take place? In the middle of highrises, a small town, on a ranch, on a boat, in a train?

xii. Utilizing assets
Readers preview your story typically in this order: cover (imagery and title), subtext, summary. Think about your summary and how it can tie in with your cover. If you summary is more abstract, maybe your cover is more literal. On the other hand, a detailed summary could work well with a more abstract cover.