The simplest method is to create small tile and then copy it all over the image. For hexagonal grid you can use tile like this:

#_____#_____##_____#_____##_____#_____#_#___#_#___#___#_#___#_#_____#_____#______#_____#______#_____#_____#_#___#_#___#___#_#___#_#_____#_____#

Then just tile this image and you will get hexagonal grid.
You can create Bitmap which will hold your tile (this image you can load from file) and then use Graphics::DrawCachedBitmap to draw tile bitmap multiple times.