When creating most node types there is a section called Description or Body into which one can put a significant amount of text or more complext HTML. When there is a reference to this node following the Browse by Category links, searching, or other forms of site navigation in which multiple targets can be listed on a single page, Drupal will show a "preview" of the node by displaying a paragraph or two from the top of the Body or Description so that one can have a better idea whether or not it is a node one wants to explore. This brings up two important points for creating optimum site content.
One is that one can insert <!--break--> into the Body or Description to tell Drupal where to make the break. This means that the Preview can be one or two complete paragraphs which provide a good introduction instead of being broken at some arbitrary point.
The other is that one generally wants to avoid putting any but very small images in the Preview portion because otherwise they will take up much more space than is "polite". Because Drupal has no ability to thumbnail or split the image, a large image at the top of the Body or Description will mean an extraordinarily large Preview. Not only does this take up much more space than other people are getting for their text-only previews, but it probably also doesn't provide much information about your node since there will be a limited amount of accompanying text. Note too that one is likely to have formatting problems because the Preview is normally displayed in a narrower center column than will be the case for the full Body or Description. So, the general advice is to put any images below the <!--break--> .
When creating your node, there is a preview button at the bottom which will allow you to see both the Preview and Full versions of your node.