Actually it is your ppi ( Pixel Per Inch) on your photo's in PSP that you need to look at.
DPI (Dots Per Inch) is only for size difference when you PRINT out your photo's, DPI tells your printer how many dots of ink per inch to put down!
DPI has nothing to do with what you see on your computer monitor!
DPI is all important to printers because printers print on paper, and paper is dimensioned in inches. But video systems know no concept of dpi at all. Video systems simply show the 412x324 pixels (PPI). Any dpi value is always ignored on the screen, regardless what value it is. Dpi is simply not defined in the video system.
dpi is simply the wrong concept for the video screen. It really does not matter what resolution was used, just as long as it was the appropriate value (PPI) to create the desired number of image pixels.
Click here to read more on this subject:
http://www.scantips.com/no72dpi.html
But as rigdonia said, you are not using images that have the same PPI/resolution, I had problems with this until I forced myself to play with an experiment:
Create an image, doesn't matter which size canvas you use, that you know is 72ppi and one that is 300ppi, just paint a little swirly thing with your paint brush on those 2, then create a new blank image at 300ppi and copy and paste both the 72ppi and the 300ppi image unto the new blank image and you will see the difference. The 72ppi image will be much smaller because it doesn't have as many pixels per inch.
hth's
Tracey