One option that you have is to check for a "small screen" device and output different HTML:
{if $_wpl.browser.small_screen}
<img src="public/small_screen_logo.png" width="100" height="50" border="0"/>
{else}
<img src="public/logo.png" width="150" height="75" border="0"/>
{/if}
There are also ways of doing this just with CSS - this can get messy and is not recommended unless you have a professional Web developer handy.