Well, I new here too, so sorry if the following is utter rubbish

...
Does the mask file have to be a .bmp file? Or what? What ever it is, normally its just a black and white image. Black means "hidden" and white means "shown". So, when the image and its mask are overlayed, anything covered by black is omitted from the final image.
But I bet you already knew all that. You want to make a mask in Photoshop though? Well, when I do it, it goes something like this...
• Say your main picture is a filled-in circle. Put it on its own layer in Photoshop.
• Make a selection of that layer.
• Invert the selection.
• With the selection still selected, make a new layer called "MASK".
• With the new "MASK" layer selected, fill the selection with the colour Black.
• If you want, you can invert the selection again (so it's back to the normal circle shape) and the fill with White.
Now you have a new layer ("MASK") which is your mask. With only that layer showing, save the file as "mask.bmp" or whatever.
Sorry I can't give more precise instructions for making selections of the layer etc... I use a Mac, so the Windows version of Photoshop might be different.
I hope that helps! :D
MadMacZed