This will probably work for making the thumbnail viewer open the files, but if PSP is the program that's supposed to open them, that feature will be lost.

I prefer to open image files with IrfanView, since it's real quick and it's free. A quick Google search will take ya right to it. But I edit the "Edit With" option to send the editing function to PSP. I don't use the thumbnail preview myself because I have some directories with thousands of photos in them. Takes WAY too long to generate the list.

RtR
PsP X2 Ult