It has been mentioned by others that using the VML option is a possibility, but it's important to note that the padding required to adjust the tab height may vary across different email clients, so thorough testing is recommended.
An alternative approach could be setting a fixed height on a table cell and utilizing vertical align bottom, although personally I have always found using padding to be a reliable method. However, I must mention that aligning content at the very bottom of a background image is not something I have often encountered - typically centering or padding from the top is more common for me.
If you're looking for a starting point:
<table border="0" cellpadding="0" cellspacing="0" style="width:100%;">
<tr>
<td background="https://via.placeholder.com/359x174" bgcolor="#00e5ff" height="174" styke="width:100%;max-width:359px;height:174px;" valign="top" width="359">
<!--[if gte mso 9]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:359px;height:174px;">
<v:fill type="tile" src="https://via.placeholder.com/359x174" color="#00e5ff" />
<v:textbox inset="0,0,0,0">
<![endif]-->
<div>
<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tr>
<td style="padding:139px 0 0 0;">
<table align="left" border="0" cellpadding="0" cellspacing="0">
<tr>
<td align="center" bgcolor="#2979ff" style="font-family:arial, Helvetica, sans-serif; font-size:15px; line-height:20px; font-weight:600; color:#ffffff; padding:8px 15px; border-radius:0 7px 0 7px;">
Earn $10 off
</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
<!--[if gte mso 9]>
</v:textbox>
</v:rect>
<![endif]-->
</td>
</tr>
</table>