Regardless of how you specify your image sizes in your KindleBook source epub/xhtml, they will be reduced to height="xxx" width="xxx" in the old-style mobi (xxx being pixels).
Code:
<p height="0pt" width="0pt" align="center"><img src="Images/image00093.jpeg" align="baseline" height="600" width="400"></img></p>
K7 only understands pixel image dimensions. I have no idea if Kindlegen is capable of conversing source percentages to K7 pixels nowadays. I highly doubt it though.