Hi,
How do you customize the upload button? If you edit the uploader's LayoutTemplate directly (right click the Uploader, then choose "Template -> LayoutTemplate" from the context menu, you will be able to replace the Upload button with an asp:ImageButton (the default is an asp:Button). You can then use whatever image you would like to use.
You can also use a HTML img element directly and then handle the image's onclick event to call the uploader's upload method. It will be something like this:
Code: HTML/ASPX
<img src="your_image.gif"
onclick="eo_GetObject('AJAXUploader1').upload();" />
Once you edit the layout template, the uploader takes it as is. You can explicitly specify styles such as width, padding, etc on the image. That way it should render the full image across browser without any problem.
Thanks!