Example: Retrieve image documents stored within a named fold

public class Example {

    /*
     * Define the name of the folder containing your images as a constant
     */
    public static final String IMAGE_FOLDER = 'Downloadable Images';

    /*
     * Example for retrieving all PNG files within the images folder
     *
     * @return A list of retrived documents
     */
    public static List<Document> retrieveImages() {
        return [
            SELECT Id, Name, Type
            FROM Document
            WHERE Folder.Name = :IMAGE_FOLDER
            AND Type = 'png'
        ];
    }
}