Разобрался, можно сделать так:
Dim pMxDoc As IMxDocument
Set pMxDoc = ThisDocument
'->
Dim m As IMap
Set m = pMxDoc.Maps.Item(0)
Dim gfl As IGeoFeatureLayer
Dim r As IUniqueValueRenderer
Dim p As IPictureFillSymbol
Set gfl = m.Layer(0)
Set r = gfl.Renderer
Set sfs = r.DefaultSymbol
r.RemoveAllValues
Set p = New esriDisplay.PictureFillSymbol: Call p.CreateFillSymbolFromFile(esriIPictureBitmap, "e:\magn.bmp"): Call r.AddValue("magn", "TYPE", p)
Set p = New esriDisplay.PictureFillSymbol: Call p.CreateFillSymbolFromFile(esriIPictureBitmap, "e:\grav.bmp"): Call r.AddValue("grav", "TYPE", p)
Set gfl.Renderer = r
'обновить вид
pMxDoc.UpdateContents
pMxDoc.ActiveView.Refresh
'<-