Well, for anyone looking for this answer, here it is:
Dim theNOD As DBDictionary = trans.GetObject(db.NamedObjectsDictionaryId, OpenMode.ForRead)
Dim rasterVars As Autodesk.AutoCAD.DatabaseServices.RasterVariables
Dim kImageVars As String = "ACAD_IMAGE_VARS"
If theNOD.Contains(kImageVars) Then
Dim rastVarsId As ObjectId = theNOD.GetAt(kImageVars)
rasterVars = trans.GetObject(rastVarsId, OpenMode.ForWrite)
' or create it
Else
rasterVars = New Autodesk.AutoCAD.DatabaseServices.RasterVariables()
theNOD.UpgradeOpen()
theNOD.SetAt(kImageVars, rasterVars)
trans.AddNewlyCreatedDBObject(rasterVars, True)
End If
rasterVars.ImageFrame = FrameSetting.ImageFrameOff
