[SOLVED] header image not showing

So I couldn't seem to post a new post and so this is not exactly about this topic but when I went to post somewhere else I was not able to and I'm getting a bit frustrated. I uploaded via Filezilla a header image into the theme folder. It is not showing up. I refreshed. Someone else said you had to add the name of the image somewhere but I don't see where that might be. I am not entirely sure my image is the right size but I don't know how to ensure that. Thank you!!