Raster as Operational Layer?

I’m fairly new to the Flex Viewer scene and have been reading the forums for help me in sticky areas. I’ve read many threads taking about problems using raster data as an operational layer, and many suggested changing the type to 'dynamic'. Where is this change made specifically? I can find the raster layer in application builder, but when I try to add it to my selected operational layers I get an error stating “Could not add as feature layer”. Where is this ‘type’ change made?
Thanks