1) Don't use bmp for showing screen shots, use png or jpg.
2) Use floats when doing math (for decimal points and division)
3) Check for and handle odd user input, like make sure the operation number the user entered is between 1 and 4.
4) Uploading multiple screen shots to a image hosting site like Imageshack (
http://imageshack.us/ ) is, imo, better than making 5 extra posts.
5) Use Alt+Print Screen to only capture a screen shot of the window, the rest of the desktop is only increasing the file size.
The icon looks good.