Borders around images

Hello,

I like the borders around images that I've put into several (static) pages. Seems to me as good old fashioned paper pictures.

I noticed that at least with jpg files this border reduces the width and height: Firefox properties of a picture show for example: "size: 400px x 266px (scaled to 394px x 262px)". Another example: "300px x 199px (scaled to 298px x 197px)". With another picture (GIF file) there is no "scaled to..." hint.

Where does this scaling come from? Is it because of the borders? If yes: Is it possible to add the borders around the picture so that the whole pixel size is a bit larger (e. g. 406px x 270px)?

My problem with scaling is that I fear it reduces image quality and it costs time.

Can anybody help me?

Best regards,
upscho