What do you want to change? If it is the header image, replace wp-content/themes/nature/images/head.gif with your own image. Be sure it is the correct size (1000×250 px). If it is not a .gif file, you will need to edit style.css and change the name at this line:
#home{background:transparent url(images/head.gif) no-repeat;width:990px;height:250px; margin:0 auto;list-style:none;}
Back up please, I can’t even find the theme options, let alone changing images. Under Appearances my choices are Themes, Widgets, Editor, Add New Themes. Header text in the upper right is right justified, wraps out of the upper band of the banner, and duplicates the page name I typed into the Edit Page title
Do I need to know that code at the bottom of your response to change the images and text behavior?
In brief, yes. I think your theme is ‘Nature 3.17.4’. It does not seem to provide any options, so modifying the code is the only way to customize the behaviour.
If that is the wrong theme, please provide a link to your site. If it is your theme, I would suggest finding another that is more adaptable.