This is done with CSS (Custom Style Sheets). The best way to have custom CSS is to have a page on your site that holds it, so whenever you update the page the CSS on the site will be updated straight away, and you will have a history of the changes so that you can revert to a previous version when a CSS change doesn't do quite what you expected.
So create a page on your site which you can call anything you like although most of us call it admin:theme or css:theme. Let's say it's css:theme as that's what I always use for my sites.
Upload the banner image to this page.
On this page you are now going to put a code block:
[[code type="css"]]
[[/code]]
You are currently using the Localize theme so the first line you must put is some code to import that theme. When you have done that the codeblock will look like this:
@import url(http://static.wikidot.com/common--theme/localize/css/style.css?0);
Now we can put the banner image in. This uses the CSS #header rule as follows and you need to set the position of the banner and whether it will repeat or not:
@import url(http://static.wikidot.com/common--theme/localize/css/style.css?0); #header { background-image: url(/local--files/css:theme/banner1hu.png); background-position: center top; background-repeat: no-repeat; }
Then save the page.
As it's the first time you've done this you now need to tell your site manager where the CSS code is that it needs to use for your site. So go to admin:manage (http://mtkiscolibrarykids.wikidot.com/admin:manage). Click on Appearance then Themes. Make sure that _default is highlighted then click the "Or use an external theme" radio button. In the box that appears type: /css:theme/code/1 and click the Save changes button.
Go back to your home page and the banner image should now be in the center of your header and the leaf image will no longer be there.
You can now make other changes to the CSS by adding rules to the css:theme page and saving the page.
If you are using Firefox, after you change the CSS in the codeblock on the css:theme page you might need to clear your browser cache and refresh (tools menu _> clear recent history then F5).
