I have 2 different sized images for desktop and mobile emails. The mobile image needs to be hidden on desktop client(ISP) but it doesn't seem to want to hide.

<img class="hideimage" style="display:block;" src="http://i.offerx.co.uk/ox/prize-entry/175/l-button.jpg" width="198" height="60" border="0" alt="ENTER NOW" />
<img class="showimage" style="display:none;" src="http://i.offerx.co.uk/ox/prize-entry/175/m-l-button.jpg" width="158" height="48" border="0" alt="" /> 

Does anyone have a solution?

Thank You

Adam Humphrey