Some email clients and webmail services such as AOL Mail, Gmail, Lotus, Outlook, Thunderbird, and Windows Mail will display a blue or black border around images that are linked and don’t have a border specified.
You can use either HTML or CSS to easily set the img border element to 0, or through CSS the border to 0.
HTML Solution
HTML
<a href="http://www.htmlemailcheck.com" title="mailmouse" target="_blank">
<img src="http://www.htmlemailcheck.com/assets/img/og-logo.png" width="500" height="300" border="0">
</a>
CSS Solution
HTML
<a href="http://www.htmlemailcheck.com" title="mailmouse" target="_blank">
<img src="http://www.htmlemailcheck.com/assets/img/og-logo.png" width="500" height="300" style="display:block; border:0;">
</a>
The style display:block; was added to prevent gaps between images in Gmail and Yahoo! Mail.
For more details see: https://www.htmlemailcheck.com/knowledge-base/remove-gaps-images-gmail-yahoo-mail/
Alternatively, to stay on the safe side you can combine both solutions.
HTML + CSS Solution
HTML
<a href="http://www.htmlemailcheck.com" title="mailmouse" target="_blank">
<img src="http://www.htmlemailcheck.com/assets/img/og-logo.png" width="500" height="300" border="0" style="display:block; border:0;">
</a>
