Okay, here is what you want to do.
Open Skin Studio, click tools at the top bar, and then preferences. Then click editing and image editors. You will see the add button, click it and then locate your PSP8 executable file. Once you have PSP added move it to the top. Then, put a check box in "convert TGA files to png before editing."
Now, whenever you double click a image it will open in PSP8. If it is a tga file, it will be converted to PNG automatically. Once you finish editing it, just click save in your PSP. Then, you need to save your WB skin and the modified PNG will automatically update back to TGA in SKin Studio with the modifications.