First, if you haven't already, take a look at the
kgPanels Wiki.
Basically, while working with the graphic as a native Photoshop .psp file, make a new layer and make it black. Move that layer below the graphic layer. Next, select the graphic (in the "Layers" pallet, make the layer for the graphic active, and "ctrl + left click" the
image of the graphic) and then switch to the "Channels" pallet. On the "Channels" pallet, make a new channel ("Create new channel" button). A new channel should appear in the channels pallet called "Alpha one". Your graphic should now look like just the black background with a dual selection Marquee around where the graphic was. That's normal. If you check your color selections (foreground and background) should have also changed to white for the foreground and black for the background. If it hasn't just change it so that that's what you have. Now change to the "Paint Bucket tool" and fill the selection with white. Turn on the other color channels, but turn off the alpha channel, and switch back to the layers pallet. Right click one of the layers and choose to "Flatten Image". "ctrl + D" to deselect the graphic.
When saving, save as a "TARGA" and be sure the "Alpha Channels" option is checked. After hitting "Save" you should get a small "Targa Options" box pop up. Choose "32 bits/pixel" and no compression, and hit "OK".
I know that PNG's can be converted to .blp's via an online application but have never done it since this method works and I can do it and trust it.
There are also size limitations for your graphics as used in both kgPanels and SunnArt, so you might want to look into that.
I hope this helps you.