Header picture problem

I'm not sure why but my header picture isn't working right. It seems to be made taller when I upload and it cuts off the top and bottom of the picture I had. I tried adding black sections to the top and bottom so that it might get centered and show the whole things better but that didn't work. I don't know if there is a tutorial or something that I need to read but I'd like to be able to put a picture in for my "header" or "background" at the top without having to redo the whole image every time I want to change it.
I was trying to do something for holidays and I don't know if I will be leaving each picture very long but I'd like it to look right when I do put one in. I was trying to find something for Thanksgiving but I haven't found anything yet so I picked something that is a mix of Halloween & Christmas that I had.
I also have been busy doing other things around the house and going to doctor/clinic appointments so I have not posted something here every day like I thought I was going to do.
Right now my back hurts after moving stuff out for the extra trash pickup the city does once a year. I was going to lay down but I got started doing some things online. I may still lay down but then I'll probably wake up in a few hours and be up all night.
By the way, I do not have problems with pictures I put in my posts, just the one for the top of my page.