Generally speaking, what you're talking about is basic html editing. They will not go in your css file, but the code for them will go in one of your skins/yourskin/ files. Exactly where the images go is up to you, but quite common is to put the images in skins/yourskin/img/ folder just to keep things organized.

There are many threads that talk about adding little bits of code (static images being an example of a little bit of code) to different parts of your blog, so have a look around with some creative searching and you should find some good info.

Also if you really don't know about adding image code to files you should probably look online for some sort of html "getting started" tutorial. There are lots of them out there for free. Oh and when the time comes, php is not something to be afraid of. All it does is create html stuff on your server. So like you would add your html code in a php file (using the proper syntax of course) and your server will process the php and add in your html bits.

Anyway I don't know of any plugins that facilitate adding an image. Oh wait: there is a header image option but I've never used it so I can't really say how it works. But it is there as part of 2.4.2 so it probably does something ;)

Thanks for the reply. If I add the image to the folder, then I just call it in the css as I would with a regular html page?

BTW, I posted in another thread, but if you have the answer that would be great: to facilitate adding an image, I figured I'd use skins with images and then just switch the image files. I uploaded the skins to the skins folder, and they appeared in my b2 admin areas as expected, and I switched my blog in the admin to the new skin. But when I went to view the blog, it says the skin does not exist and must be changed in blog properties (where I did change it).

Any idea whats up with that?

thanks again!

I guess if you're doing a background image then yeah adding it to the css file will work. Your original question was all encompassing about images in general, and css will only provide background images. For anything else it'll be html in one of the skin's php files.

The other topic is in the other thread.

In version 2.4.2 you have the 'Logo widget' that does something similar to what you want, but it's not popular.
You described the correct actions: upload an image, change css like you would with normal XHTML. The css varies with skins but looks similar to:

#headerimg {
	background: transparent url('img/headerimg.jpg') no-repeat center 15px;
	margin: 0;
	height: 180px;
	width: 100%;
}

*edit*
That's an header image, background image from this particular skin is:

#page {
	background-image: url(img/bg.jpg);
	background-repeat: repeat-x;
	background-position: left top;
	margin: 20px auto;
	padding: 0;
	text-align: left;
}

Good luck