Making icons for the Sidekick is the same as making icons for any other purpose. Icons are just small images used to represent a program, file, and etc. therefore any image can be converted into an icon. The only difference between regular images and icons is that regular images do not contain an alpha channel. The alpha channel is used to represent up to 256 levels of transparency. When you add the alpha channel to a regular image you can make parts of that image be transparent or semi-transparent. If you look at the default instant message icon image on your Sidekick you will see a yellow smiley, but if you open the image in Photoshop or GIMP you will see the yellow smiley inside a squared transparent background. If you were to save that same icon in JPG or BMP format the transparent background will become white, that is because JPG and BMP do not store alpha channel information.
I've seen many themes posted with icons with a squared black/white background, that is becuase they have uploaded JPG or BMP files to the theme creator on the SkLX/SkSlide websites. The theme creator does not remove backgrounds for you, so do not expect to get an icon with a transparent background from there unless that's what you're sending. Formats that support the alpha channel include PNG, ICO, PSD, GIF and others, for Sidekick themes you only need to work with the PNG format.
In the video below I will show you how to add the alpha channel to regular images so that they can be saved in PNG format and used as icons. Part 1 of the video shows how to do it on Photoshop & Part 2 shows how to do it on GIMP. If you're watching this video I assume you already know Photoshop/GIMP basics and I will not go into detail about resizing and fitting the images. This video is no way near a tutorial on how to extract images (remove backgrounds), there are hundreds of ways to extract images, If you do not know how to extract images, I suggest you watch videos on how to do it because in this video I'll use the simplest method which does not necessarily yield the best results. If you're watching this video to learn how to make icons for your Sidekick themes, you should download one of the theme templates below:
The icons made in this video can be found in my PACMAN theme.