To properly align images in SiteExecutive set the class to block_left or block_right within the "Image Properties" dialog box when importing the image or by control or right-clicking an image on a page. (See image below for where to find this setting.)
DO NOT Rely on the SiteExecutive "Align Right" or "Align Left" controls in the toolbar. This will not create the correct amount space around left or right aligned images.
See also Using Images in the Web.