You can use the draw order command.
Type dr on the command line ( this is shortfor Draw Order)
You are then asked to select a component e.g. line, block, or in this case an image...so...select the raster image you have inserted...press return
It then asks you where in the draw order it should go, in front of another object, behind another object, generally to the front or generally to the back, in this case by hitting return you choose the default which is 'generally to the back' .
So to summarise,
type dr ...hit enter
select image...hit enter
hit enter again
hey presto the CAD line work now shows over the image.