Do you want to know how I edited my halloween-makeup so that it will be fitting my avatar in Opensim? Maybe someone will add some help with the renaming of the files...
Okay, this is what I have done:
1. Open Makehuman (folder) -> data -> skins -> textures
2. Choose one of the skintextures and import it into your picture-editor, GIMP for example (you will find lots of tutorials for gimp on youtube, e.g. the magic scissors, so I'll not write a tutorial around gimp here). The skintexture is .png 2048x2048 px, so it is possible to enlargen it for better results (I used 5000x5000px). Rotate it to simplify your workflow.
3. Now add layers for eyes or lips or draw lines to paint your makeup.
4. Save your skin in a way that will allow you to work on it later to correct the result.
5. Resize your picture to 2048x2048 px, rotate it and save it as .png ->
6. -> save your picture (the edited skin) in the original skin-datafolder (MakeHuman) and name it exactly the way it is called in the texture-folder, example: 'old_darkskinned_male_diffuse' (deactivate the original skin with this name by deleting or renaming it)
7. Next time you choose the material-tab in Makehuman, your avatar should wear the new created makeup (by selecting 'old african male', example related to the skin in step 6). And you see exactly where you will have to edit parts (mouth too large, eyes to small...) Edit as often as it is needed in your picture-editor, and finally you'll have your skin prepared for your upload to Opensim.
The eye-texture was changed the same way.
Another following step will be to separate head and body, because the texture-upload in Opensim and SL is limited to 1024x1024 px max. With the head separated it will be possible to create more details in your face.
