Not sure if I understand the problem; is it that your image editing software won't open the image, or that you're not sure how to send the image to your editor?
You should find you image in the 'Files' paned in Dreamweaver: siteassistpro_images > contentWrapper_bg.jpg (or whatever name it has).
If you have associated an image editor with images or image types in your DreamWeaver Preferences (Edit > Precerences > File Types / Editors) then you should be able to right-click the image in the 'Files' menu and select 'Open with' (To see listed image editors or browse for one) or 'Open with Photoshop' if that was installed with Dreamweaver.
In your image editor you can then adjust the image and, depending what you want to do, i.e. add text, overlay with your own image etc.
The images that SiteAssist Pro provides when it creates your pages do not have editable text. You would have to add text to it using your image editor.
If you meant something else, please could you post back with more details?