This is related to how Microsoft Outlook 2013 renders HTML and CSS. The same email may appear correctly in Microsoft Outlook 2010.

In order to resolve the display discrepancy with Outlook 2013, add the following code to the <table> tag that houses the image:
  • style="mso-table-rspace:5pt;border-collapse:collapse;border:1px solid #ffffff;
For example:

<table align="left" border="0" cellpadding="0" cellspacing="0" style="mso-table-rspace:5pt;border-collapse:collapse;border:1px solid #ffffff;">

Please note it is always best practice to make a backup the HTML code before editing or adding code for the email.