If you want to animate the spin, to show the "reels" spinning, you need to do your own drawing. If you're working the GDI, then the key call is BitBlt (or one of its 'Blt relatives, like StretchBlt)
And if you do your own drawing, then you'll need InvalidateRect() to force a redraw so the spinning works.
But if you just use WM_SETBITMAP to swap a SS_BITMAP-style static control's bitmap, then it might not be necessary (I'm not sure here. The rules might be different for a static control in a dialog and one you creates yourself as the child of a normal window? As you're using a dialog, I don't think it's required.)
PS This example shows how to load .bmp file and then render the loaded bitmap.
HOWTO: How To Use LoadImage() to Read a BMP File
If your bitmaps are small, you could store them in the app's resource section and load them from there, rather than a file. Or even create them in memory.