You have to use the transparent color, which is #0 on the palette. It is in the grayscale color range, though some color scripts may intentionally avoid using it. If you can put in your own image, that'll be easiest (make sure it has the correct palette); just set your transparent area to color #0. To do it with the .SPK files, you'd have to command the game to display it with the grayscale color range, and it'd have to have the brightest color where you want the transparency. I'm not sure but I think you set a color offset (in multiples of 16--one color segment) which will vary depending upon the base color of the image. It's difficult, so I recommend just adding your own images to the game. They must be .png or .gif (preferably .png) and they must have a 256-color palette. If you copy an existing similar image from a mod, you can get the correct palette that way, though keep in mind there are multiple palettes used in different parts of the game. You'll need an image editor that is able to work with 256-color palettes, such as Irfanview, ASEprite, MTpaint, or GIMP. Photoshop and MS Paint will not work.
