It will be more easy to put an image as button if that serves the purpose.
<img id="myOddShapeButton" src="~/buttons/myOddShapeButton.gif" alt="myOddShapeButton " title="My Odd Shape Button" style="cursor:pointer;" onclick="javascript:myOddShapeButtonClick();"></img>
If this doesn't fulfill your requirement, then I believe you will have to create your own button control.
Creating a Custom Rollover Button in ASP.NET[
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http://net.tutsplus.com/tutorials/create-asp-net-server-controls-from-scratch/[
^]
And there are many more article if you search.
Hope that helps
Milind