Is it possible to use use my own buy button image?

If you want to use the managed templates included in ASA 2, there is no option to use your own button image currently. But you can duplicate a managed template or create your own from scratch and use whatever button image you like. The HTML code for this could look like this:

<a href="{{ AmazonURL }}" target="_blank" rel="nofollow" title="{{ Title }}"><img src="path/to/your/button_image.png"></a>