Sorry, I've been asleep. It's strange how we humans need that sort of thing.
It seems like you've had quite a bit of drama since I've been away. Yes, I've had trouble with PSP8 not saving the PNG file properly too. Then, when it is transferred to TGA it is messed up. You'll want to check your PNG options. I don't really use PSP8, but I have it. I haven't even re-installed it since my system upgrade. I've just been using PSP7 because it doesn't have those sorts of problems and I like the way it works better and I'm use to it.
Instead of clicking save, try clicking save as, then look at the options for your PNG file. Try fooling around with changing the options a little, then saving it and saving the skin studio changed file. If it doesn't save properly, then you need to right click on the image in skin studio and hit cut. then double click the image again, and a new one will apppear. You can keep changing the PSP PNG options and applying the changes in Skin Studio until the TGA file is made properly from the PNG file. Then, you won't have to change the options again, and you can just use the save button. The options won't change unless you change them.
If you can't get it to save properly, let me know. But, give it a try yourself first. If you can't get it to work right yourself, I'll re-install PSP8 and give you a step by step and the right options that you want in PSP8.