Add default image border

A line of CSS will do the trick:

img{
border: 2px solid white;
}

You must select the tag that hold the posts. Check it with your browser debugger and write it. Could be something like that:

.entry img {
  /* your code */
}

Remember, don’t copy-paste it, check what is your entry tag!

I can not tell you what is your theme’s post wrapper CSS class to give you exact tip. Can you link me your site, this will greatly help. (and please, next time you have such questions link the site if its hosted somewhere)

It can be styled like that:

Select all images that are not floated to left or right, but you could just add the stylings for this with theWordPress CSS classes

img.alignnone {
  /* your code */
}

or

reach it by the way the theme is structored:

#container #left img {
  /* your code */
}

The first one will select all images from all posts and apply styles to them and the secound will select all images from the div#container #left and apply styles to them.

This is all you can do with the current theme structure. If you have more questions, ask 😉