custom signature

No pb. To insert the signature, place this code in your functions.php file, before the last closing php tag, which looks like this: ?>:

function custom_insert_my_signature($content) {
    if ( is_single() ) {
        $sig_url = 'insert URL to signature here';
        $sig = '<div class="signature"><img src="'.$sig_url.'" alt="signature" /></div>';
        $content .= $content.$sig;
    }
    return $content;
}
add_filter('the_content', 'custom_insert_my_signature');

You can then style the container and the image from your style.css file.
To target the container:

.signature {
properties go here;
}

To target the signature:

.signature img {
properties go here;
}

Don’t forget to paste the URL of your signature in line 3 of the PHP code below.
Cheers!

Try this in your stylesheet (style.css):

.signature img {
border: 0;
}

Cheers!

Try this then:

.signature img {
border: 0!important;
outline: 0!important;
}

If that doesn’t work, could you please share a link to the page in question so I can take a look?

The border seems to be coming from your style.css file:

#content img, #contentwide img {
    border: 1px solid #D0D0D0; /* delete */
    float: left;
    margin: 3px 10px 3px 0;
}

So you could delete the border property in those selectors. However, if you want to keep your borders in your images but remove them from your signature, do this:

#content .signature img, #contentwide .signature img {
border: 0!important;
}