That will work well for TS12, T:ane and TRS2019.
The thumbnail can be either a .texture.txt and a .jpg in which case the reference from the config is to the .texture, or it can be just a .jpg and the reference from the config is to the jpg - your choice. The image size should be 240x180 and the 'width' and 'height' tags should be 240 and 180 respectively.
That might have been referring to the diffuse image alpha channel, but the description can be confusing. Of course, an alpha channel isn't a color, so 'white' or 'black is meaningless. For your diffuse image the alpha channel is either excluded (ie, the image is 24-bit RGB) or is 255 (sometimes referred to as 'white'). However, 255 is provided by default, so there should be no difference between excluding it and setting it all 255. This might be another example of out-of-date advice.
The other possibility is that it refers to the normal map alpha channel (mentioned above) which should have a value close to 0. Otherwise you get excessive reflection when viewing it towards the sun. (Note that this is a per-pixel setting, so you can produce some very interesting reflective effects by creating the alpha channel from the image, such as icy patches on a road, but that's a topic for a later occasion).
GIMP is extremely difficult to use and does a lot of things that are not compatible with Trainz images. I would recommend Paint.Net. https://www.getpaint.net/download.html However, be very careful about selecting your add-ons, as it is easy to be overwhelmed by the variety and functionality available. Only install what you really expect to use - if the menus run off the bottom of the screen you probably have too many.
I haven't used Materialize but have heard many good things about it.
