There are two categories of "Phd" software. One category is the type that is so intuitive that the software almost tells one "Push Here, Dear". Unfortunately, GIMP is in the other category of "Phd" software, the kind that almost requires a PhD degree to be able to use it. It's a complicated bit of software, and it takes most people a period of time to learn to use the software.
Unless you downloaded GIMP from the official website (
http://www.gimp.org ), I suggest uninstall the software, and download the package from that website, There are nefarious sites out there that take some open source packages, and insert malware. But even if you choose not to do that on the GIMP website, the "Documentation" link in the upper right corner of the site links to a page where the second heading from the top is "Tutorials", with a link to "Tutorials on GIMP". Folloiwing that link will take you to a page which has beginning, intermediate, and advanced tutorials on using GIMP. Start working through some of the beginning tutorials to learn to use the software.
There is also a WIKIbook on GIMP, at <
http://en.wikibooks.org/wiki/GIMP> which can be helpful.
Once you know how to use the tool, you'll need to know how it is used to reskin. A "skin" is an image texture mapped to a mesh so that the mesh will display the texture. Reskinning is the art of taking the original creator's skin, and changing it so that when it is in game, it is a different object. To do that, you need to know how the original skin is applied to the model, so you know what to change. This is not always easy, because experienced reskinners will pack the items they need for the skin tightly into the image file, and map the file to the mesh in ways that are not always straghtforward. Relatively small parts of the mesh may be mapped to much larger parts of the skin, and large parts of the mesh may be mapped to small parts of the skin. And the same part of a skin may be mapped to multiple parts of the mesh. For example, in a house, all of the windows of the same size may be mapped to a single window image on the skin; on a box car, both sides of the car may be mapped to a single side on the image, and on a locomotive, all of the radiator fans may be mapped to on instance of the fan on the image. And there are some, and will probably be more, items which cannot be reskinned because they are colored without textures.
After you have some experience with GIMP, choose some simple structures, open them in for edit in explorer, and look for texture files. The texture files will be files with an image format, typically having a "tga" file extension. Copy the image file(s: there can be more than one texture file on an object, particularly if it is a complex object) into another location on your computer, and close the asset. Then, install that asset in a test baseboard and examine it closely from all directions. Open the image(s) you copied in GIMP, and compare what you see on the test installation with the images you copied from the file Try to identify to which bit of the image each bit of the Trainz object is mapped, but be aware that it may not always be immediately obvious. When you've done that, you'll know what the effect will be of changing a particular bit of the texture image.
Summarizing the essential steps: Learn to use the software you're going to use to reskin; learn to recognize how a skin is mapped, and change the skin. But of the three steps, only the third one is guaranteed to e simple, since that, after all, is only a matter of copying a file.
Final bit of advice: start small. Don't start by trying to reskin a locomotive. I'd start with a small structure, then move on to increasingly large and complex ones, then look for freight cars with simple skin [Note:what appears to e a simple freight car, may not have a simple skin. If I do a flat car it will have a complex skin because every the required writing has to be in a small space; conversely, what seems to be a complex car, like a tank car may have a simple skin. then locomotives.
Final Final bit of advice: don't expect to be a top drawer reskinner by the end of the month, or even the end of three months from now. It will take time to learn to do, and more time to learn to do well, but it's a skill that is in demand in the community.
