Unable to remove image border

If borders are being added to images by your theme, then you can usually remove them pretty easily with CSS. Here is a very basic example:

img {
     border: none;
}

You would need to replace “img” with the right CSS selector for the theme you are using.
http://en.support.wordpress.com/custom-design/how-to-find-your-themes-css/

Or if you’re looking for a way to edit margins and borders on a per image basis, here is is a new plugin that adds options for those things:
http://wordpress.org/plugins/advanced-image-styles/