Please forgive me for being a bit confused about exactly what you are trying to do. It would help me to know what to suggest if I had a better understanding of exactly what you are trying to do.
These are two very different things and how you would accomplish using a background image (I assume you mean a "static" image over which the "text" is written) would have to be accomplished in two very different ways. For one thing, a Joomla! "section" is only a classification mechanism and is not "content" in and of itself so you cannot use a traditional "background image" declaration with it (some significant template tweaking with CSS and custom classes would be involved). A Joomla! "content item" on the other hand is actually only a database row containing the "content" to be displayed in the appropriate place in the template and you might could put a background image inside a div within the content to accomplish that (though not with the WYSIWYG editor, and it can't be done by "pasting").
Are you talking about the "Edit CSS" link in the Site -> Template Manager -> Edit Css menu structure, where you are taken to the template's CSS file to edit? I'm sorry, but while I am very familiar with Joomla!, can't visualize what you are doing and/or what you are trying to accomplish. In this case, given the terminology and the complexity of Joomla!, it would help a lot if you would detail the keystroke that got you to where you are doing this "editing". For instance, "After logging in to the backend/admin section of Joomla!, as an administrator/user/whatever, I select All Content Items, click the link for the item I want to edit, and am presented with a WYSIWIG editing screen (telling me which editor you are using would help). Then I ...", etc.
Without a better understanding of what you are trying to do, and how, it is hard to point you in the right direction - it depends a lot on what you are tying to "embed" HTML in, and where you are trying to do it.
Joomla! does a very good job of "protecting you from yourself" when it comes to the separation of "style" and "content" and exactly how/where to make certain changes is not necessarily trivial to sort out (and in many cases, that is a "Good Thing(tm)").
For instance, without disabling the WYSYWIG editor when entering content, some HTML you might enter (even using the "html" entry mode of the editor) will not be saved (the editor will "protect you from yourself" and refuse to save/rewrite it!).
Maybe if you provide a relevant link to your site (the page/link you are trying to modify), and indicate in the post what you are trying to change about that page, and how you are trying to change it, I'd have a better idea of how to offer useful help.