Changing something in the CSS of the theme (if you have the paid CSS upgrade) will apply to all instances of that something. To add borders to images selectively, you need to do it manually in the html post editor. See here: http://wpbtips.wordpress.com/2009/10/14/borders-pt-1/
ok… now i’m having a problem. No matter how small the picture is, when I use the < div > code in the link that you posted the border goes across the whole post content box? Any hints I what I need to do?
You didn’t read the post carefully enough! Div tags are for sections, not for images. For an image, you modify the image code itself, for example change this: <img src=[etc]
to this: <img style="border:1px solid #2f4f4f;" src=[etc]