If it's helpful for anyone using this tutorial I've built a web page which allows you to do the conversion in your browser, then download the files individually. The benefit of this is that you can use any file format your browser supports, bust most importantly: bmp, jpg, png, and even svg.
Once you've loaded the image click each of the images to download each tga.
Once you've loaded the image click each of the images to download each tga.
GoldSRC Steam Background Image Converter
For now, the tool is set to output at 1920 x 1080, in sets of 256 x 256px, uncompressed 24-bit TGA images.