Hi Dave,
Yes, you could have one button that first submits the image(s) to SharePoint and then submits your form. That would avoid using two buttons. However, as you point out, the image upload could execute, then the xml submit fails.
I would suggest looking at our ImagePicker XTP too: http://www.qdabra.com/en/products/QdImagePicker.aspx
This XTP allows the user to select an existing image or upload a new one. So you are unlinking the image submit and the form submit, but you give users the added bonus of using an existing image.