Answer
The SaveAs method supports the following raster formats since ArcGIS 9.2:
'JP2' - JPEG 2000
'JPG' - JPEG
'PNG' - PNG
'GIF' - GIF
'GRID' - ESRI GRID
'IMAGINE Image' - ERDAS IMAGINE
'TIFF' - TIFF
'BMP' - BMP
Note:
The name strings are case-sensitive.
For the latest strings, see the link in the Related Information section below.