If you're having trouble with black backgrounds on your PNG images, here are a couple tips...
#1: make sure you're saving the file to your disk, not copying and pasting it. Yeah, I know you can always remove the black, but it's never quite the same. The file coming from your hard drive and either loaded or dragged & dropped into your PSP will be less likely to have a black background...
Especially if you do #2: In your PSP (v. 7 doesn't have this option, [or ANY that I could find], sorry! :-\) go to file/preferences/file format preferences, and click on the tab for PNG. Make sure the radio button for "load and save transparency to/from layer transparency" is checked.
This is the same process in v. 9 & X. Since discovering this, I haven't had a black background yet on any image file. :)
Just thought I'd pass it along.
xoxo ~ Kirsten