Hi mailgirl,
In order to upload a logo that combines an image with text for your Auctiva store, you would first need to use some image editing software to overlay the desired text over the image you'd like to use, then save the product as a new image file. You could then upload that image as the header image for your Auctiva store and the text would be there.
I haven't done much image editing or logo creation myself so I can't really offer you any specific advice as to how to create your logo. Perhaps another forum member will come along with some advice that may help you though.
If you have any additional questions or concerns, please contact our customer support team using the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike