i'm not sure i know exactly what ur referring to. but the way i see it, you have to design your entire page, and then make the images where they fit. the point of the images is to be part of the webpage, if you do the images and then try to make the webpage, you'll probably end up having to modify your image sizes anyway because they won't line up smoothly.
however, if you are talking about using FW for the image layout before DW then that makes sense because in FW you ARE designing the layout with the images as a part, then you can just do the add tool with DW and it's all very simple.
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