Adding a background color, a small margin and a border should frame any image placed into your blog.
Unless you have some funky css forcing something else to happen.
For a universal img tag solution add this code to your style.css:
img {
padding: 5px;
background-color: #FFF;
border: 1px solid #333;
}
see what happens.
If you desperately need to keep whatever else your design requires for other images you can solve the trouble in one of two ways:
class the css for your entry section only:
.entry (or contents, or whatever your index declares) img {
padding: 5px;
background-color: #FFF;
border: 1px solid #333;
}
OR make up a new id and wrap the images you want to have borders in that id when you want to:
#wrap img {
padding: 5px;
background-color: #FFF;
border: 1px solid #333;
}
The markup for this would be:
<img id="wrap" src="whatever.jpg" alt="" />
If none of that helps, then I have no idea what your trouble is…