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The images you begin with must be RGB images. This means that there are three channels in the image, which contain all the color data in your image: Red, Green, and Blue. These are the primary colors of light, which your monitor can mix together to create every other color, shade or hue. Some image formats (such as the TIFF format, which is the format recommended for plug-in images) will also support additional channels, called Alpha Channels. Alpha channels are shown in greyscale. An alpha channel acts as a mask for your image, where it is black, the image is masked out (you can't see it), where it is white, the image shows through. Where it is grey, the image will be shown semi-transparent; the darker the grey, the more transparent the image will be. This is how you can take an image of an object (say, a spaceship), and create a plug-in where only the ship appears.
First of all, you must select the ship. If you have a solid-colored background (black works well), you can do this by using the Select > Color Range command, and clicking in the black background, and then Select > Select Inverse. If you prefer, you can use any other means you want of selecting your ship, but this is a quick and easy way.
Next, you must create your Alpha channel. In the channels palette of photoshop, there are a series of small buttons along the bottom. One of them has a rectangle with a circle inside. Click it, and you are done. An alpha channel is added, masking out everything that is not selected; in this case, everything but the ship. This method works well if your object has hard edges. If the edges are soft, or feathered, however, try this method.
After you have selected your object, create a new alpha channel by clicking the New Channel button in the Channels palette. (It has a page with a bent corner on it) This will give you a new channel filled entirely with black. With your Object still selected, get a paintbrush and paint your selection white.
I'll try to add to this in the future, so if you need anything else explained, let me know.