MI-L Converting raster to grid

[Richard Hill]

Hello Folks,
 
Is there a simple way to convert a registered raster  image to a grid? I
have a digital terrain model of the Murray Basin, but only as a JPEG. I
wish to be able to apply high/low cuts and stretch various ranges of RL
to exagerate certain Topo features.
 
Any advise would be appreciated, even if it is a "No, it cannot be done"
 
Cheers to all over the festive season
 
Richard Hill
Project Geologist,
Southern Titanium