What Melanie said for the dirty skin look.

As for why bum files are so big, they are an uncompressed file format similar to bmp. When you load them, that 12mb file will take about 12mb of active memory. With a compressed file format like jpg, the file many only take up 200k on your hard drive, but when loaded it may also need 12mb of memory for your computer to use it. In other words, think of jpg file compression as "disc storage compression method". That disc storage size isn't a direct reflection of how much memory the image will use when its uncompressed and loaded into active memory by your computer.

Making the images physically smaller is really the only way to make them use less memory. A 4000x4000 image needs a boatload of memory. A 2000x2000 needs much much less, but may still be too much for your particular computer system to handle when combined with the other textures needed for the scene.

As for inverting bumps, when making them myself using the texture as the tool I do sometimes get better results by doing the grayscale and invert method and sometimes I don't. I don't know why that is, but it only takes a minute to try both and I go with whatever looks best. Sometimes I reapply the full color texture "as is" to let Poser make a bump map from it and it works OK too. shrug