There is an overloaded version of Blt that takes a GDRect destRect param.
To change the rotation centre, use the BltOptions.GDBLT_ROTATIONCENTER flag, and set the cetnre point in the GDBLTFX structure.
BLtFast is faster, but supports fewer options (such as rotation and scaling). You should use BltFast whenever possible.
Take a look at the GD help at http://www.gapidraw.com/docs/gapidraw/301/index.html
It is for c++ but gd.net is basically the same except for name changes (CGapiSurface == GApiSurface etc)
