Jefferson,
Just so you know and that is does not become a point of confusion in the future. The Flex Viewer as an application that is built using the AGS Flex API. When someone asks if you are working with a custom API application or the flex viewer, they are trying to determine if you have Only used the AGS Flex API components and built a Flex Application from scratch, not just customizing the viewer.
So, I understand your issue to be that you are loading a image service as your first basmap and thus it is using your image services WKID to set your maps spatial reference. So my fist question would be are you using a tiled basemap at all in your flex viewer and if so why are you not loading that service first? Are all your services (i.e. image service and tiled map service) in the same WKID?