Could you please be a little more specific as there are many sides to WordPress. For starters, you can find a lot of information about WordPress right here.

You are very welcome. I’m trying to construct more posts to help people like you with the start up of a blog in the future.

All the best

You can never ask any stupid questions (or basic as you mentioned) since the questions not asked are actually the dumb ones.

To change your header you need to log into your WordPress admin and navigate to “presentation” then open the style.css (normally the first one) which is the style sheet of your blog. Somewhere in the there is the code that determines the size and position of your header. It will look something like this but could vary widely depending on your theme.

#header {
background: #0482cd url(images/headerbg.png) repeat-x;
width: 100%;
height: 142px;
}
As you can see, the URL of this example header points to the name of header image (which is in png format but jpg is widely used and works the best for non graphic artists).

Further you also see that it is sitting in the “images” (of the actual theme you are using) folder in the domain.

To replace you simply create a new header with the same dimensions (header width is explained also in the style sheet but maybe a little further up)

You then upload your new header image (best named like the original) via FTP to the relating folder. (in our case “images”) It will ask you whether you want to overwrite the existing one on the server and you say yes.

Then open your browser or hit refresh to see the new one.

A note: it always pays to save your exisitng files on your hard drive if you tweak with codes so if things go wrong, you simply go back to the last one used.