Transparent Monster Texture

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Assuming the texture doesn't have an alpha channel, how do I make it translucent?

Well, I only know how it's in in Photoshop: simply select the only layer that should be there when you open the picture and drag it's opacity slider to 50% or something. That should automatically create a half-translucent alpha channel when saving as a format that supports alpha.
If it doesn't do it automatically, go to the channels tab instead and click "add channel", which will create an alpha channel. Fill that channel with grey - the darker the grey the more translucent it will be. Gimp probably works similarly, never worked with it though...