Use an image
type input:
<input type="image" src="https://stackoverflow.com/Button1.jpg" border="0" alt="Submit" />
The full HTML:
<form id='formName' name="formName" onsubmit="redirect();return false;">
<div class="style7">
<input type="text" id='userInput' name="userInput" value="">
<input type="image" name="submit" src="https://jekyllcodex.org/uploads/grumpycat.jpg" border="0" alt="Submit" style="width: 50px;" />
</div>
</form>