The optimizing command reduces the pictures that are in your .web file and not those on your website. Optimizing those pictures is done upon export, so make sure that in the Utilities/Web Properties/Website dialog the "Retina (hi-def) images" is ticked if you want your pictures to look good on hi-def devices.
If you want high-quality images to show on your website, also make sure that in the Utilities/Options/View dialog the "Import at dpi specified in the image file" radio button is ticked (or otherwise the "Resize large imported photos to xxx pixels" radio button with the appropriate pixel value).
However, as you mention that you use 'loads of photographs,' this may slow down your computer (hence the "Optimize all images" command). You also mention that you're using a supersite template (which means one large page) so with 'loads of photographs' all these images need to be loaded before the browser will show the page. This may negatively affect user experience when visiting your site. So, it depends how many is 'loads of photographs'.
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