Check the TypoScript object browser. There you will find the object tt_content
, which contains the rendering definitions for content elements. The rendering definition for images is found at tt_content.image.20
, for example
tt_content.image.20.imageStdWrap.dataWrap = <div class="csc-textpic-imagewrap" style="width:{register:totalwidth}px;"> | </div>
The default definitions given there are usually provided by the static TypoScript of CSS-styled-content. You can overwrite them in your own TS, but when updating to a newer TYPO3-version, the default template may change, which could result in additional wrappers.
Update
Most content rendering in TYPO3 is defined in the TypoScript object tt_content
. You can browse all TS-objects that will be used on a page by selecting the "Template" module and the page in question, and then choose "TypoScript Object Browser" in the selectbox at the top of the window. To understand what that stuff means, knowledge of TypoScript is necessary (Tutorial, Reference).
You can add your own TypoScript, which may override existing settings. You can do that in the Template-module too, but usually this is done by creating a file containing the script somewhere in the fileadmin folder and including it from the Template module.
The above enables you to edit the markup of the page. (Additional) CSS is usually defined in external files, that are included by a PAGE object (see the reference about that).