"I know it's impossible to do but i'm asking anyway. " Very commonly done. There are two formats that support transparency, .gif and .png. "Save for web" in photoshop into one of those formats and you should be able to do what you want.
I want to be able to see through an image of glass. I know that i can make the one color of my choice to make an image tranparent, but in the case of glass, i want to be able to see the background underneath the picture NOT surrounding it like aroun a drop shadow or something.
What you can use is the 8-bit (usually enough, depends on the palette and image) alpha channel for a PNG image. Windows IE doesn't support this rendering, but other browsers do (Mozilla, Opera, even IE for the Mac). The only cross-browser method is using a plugin, like Flash, to obtain these effects.