Ok I'm going to try explain you. It may not seem easy so feel free to ask for details.
First you should have the two folder "exe" and "romfs". We will be working on the latter.
*Download GARCTool here
*Download bclimtest here
*Download png2bclim here
Now you want to open GARCTool and select the FILE with the mugshots (a/1/6/0), hit process and wait
Once finished go to romfs>a>1>6 and you should see a folder named "0_", open it.
There you have all the trainer mugshots in a .bclim format.
Open bclimtest.exe and navigate to that folder to select a file (mugs start at dec_006) and then tick "save png" and hit "Go".
Edit the png with whatever sw you want and save the new image in a png format.
Then open up png2bclim and re-convert it to a bclim format. Be sure to name it exactly as the original ("dec_xxx"), then you can replace it in the "0_" folder.
Once you've edited everything you want head to GARCTool again and select the FOLDER "0_", then click Go and wait.
You can now delete de "0_" folder and the old 0 file in a/1/6 and rename the newly arrived "0_.garc" in "0" (ONLY 0, no format, no anything else, just 0).
Done, you can now rebuild your romfs and play with custom mugshots.
Hope it helps.