Background Image Diffculities; Newb

Alright I had a thread here yesterday, but I figured out all my problems. I then went to create my background wall paper to place into the background.

Everything worked out well, except one thing. The background or forum cuts the wall paper off at the bottom. I would rather have it cut off whatever is left from the top.

https://i.servimg.com/u/f48/12/38/81/20/skies11.jpg

This is the wallpaper I've been working on, just so you have an idea of what I'm working on/trying to get working on the forum.

See I would rather have it show all the mountains that I worked the most time blending and fusing then to cut that all out and just show gradients of blue. For all the work I put into it, that's disheartening.

This is my forum, in case you need to reference it to see what is happening to the wall paper.

http://www.avatarepisodesrp.com/

Is there a way to mend this situation? Some kind of code or perhaps something in the admin panel that I missed? Thanks in advance for any help!

***In short: Is there a way for me to be able to position an image (serving as the background) at the bottom?

Find in CSS body {

and put height:1000px; for example and tell me how it worked.

I tried it with and without the body function, but it didn't work. I'm supposed to do this all within the Pictures and Colors CSS section correct?

I'm on phpbb3, by the way. When I enter the css style sheet within Pictures and Colors there's nothing in it. It's blank. Thanks though!

This is what I did body {height:1000px;}, but it didn't work.

You should make that background image smaller in Photoshop. You know to do that?

Yeah, I can change the image size or canvas size. I did that a lot of times when experimenting with the forum lol.

The reason I kept making it larger was because the bottom where the mountains are kept ending before the bottom of the forum page. It was like right in the middle of all the forum content and then it just showed all white for the bottom.

I'll try that again. I think it was something like 20x40. Would changing the size in photoshop or in the CSS actually make it more likely to be positioned at the bottom? It just doesn't sound too likely to me, but I'm willing to try anything.

Edit: I was doing some research and I found this thread.

https://help.forumotion.com/t91715-how-can-i-past-bottom-image-into-my-forum-code-without-ruin-it?highlight=position+background+bottom

This is the code they used, but it's for PunBB

#pun-foot {
background-color: #FFFFFF;
background-image: url("https://i.servimg.com/u/f21/14/71/02/97/down_f14.jpg");
background-position: center bottom;
background-repeat: no-repeat;
height: 400px;
margin-left: -128px;
width: 1200px;
}

^ Is there a way to do something like this for phpbb3? If I can't work the way I have been, I wouldn't mind being able to insert just the mountains at the bottom background banner section and then just tweaking several gradients of blues for the background...thanks for any help/advice!