In a HTML document images are not really part of the document (like layers in a Photoshop document). The HTML code use links to bring together images so if you delete an image linked in a HTML document the page will still be visible or editable only that you will not see the deleted image in the page.
When you want to replace an image the image new image will automatically appear in the HTML document. But it is very important that the new image to have the same width and height with the replaced one because if they are different the new image will appear distorted (shrank if it is bigger that the one it replaces or enlarged if it is smaller).
In Dreamweaver you can adapt the table to the new image if it is smaller or bigger but this could change dramatically its appearance and this is not a pleasant situation. Below you have an example of a web page that has an image larger than the previous (the images are marked with red). Notice that the layout is completely modified (the right area marked in red). Note that in the Properties palette the width is bolder and a circle with an arrow appears. To let Dreamweaver adapt the table to the new image you will only have to press the circle with arrow icon:
If you modify something in the Photoshop layout but you don't modify the slices, when you'll export again the layout the changed images will have the same size with the old ones so you will not have problems.