Help with image borders…

I am trying to have borders around the images I use for “posts”, but not to images I am using in “pages”. I am happy with the image border default in the .css of the theme I am using but don’t know how to turn it off for the images in the “pages” section of my site. I tried turning border=0 in the .css which did turn off ALL borders throughout the side, then tried to specify borders=1 for the images I used for “posts”. This did not work. I do not know if the master .css for the theme overrides any indiv. settings. Can I specify a different .css in the code secion where I write my posts? If yes, can someone please leave an example of what the code would look like? Thanks in advance!