if your canvas is 600dpi - and you pull in a 600 dpi image as one of the layers and stretch it, the image will still be 600dpi, but may become grainy/pixelated. It's always better to scan at a higher resolution than to stretch by any great amount. The end result will always display the DPI of the actual canvas, not the layers you have imported. This is why we can't work with low quality scans, as minimum dpi requirement for the overall image for the site is 300.
So even if you find a lovely clean 96dpi logo, once it's stretched to sit nicely in your final cover, photoshop will report the overall image as 300, but your lovely clean logo will suddenly seem grainy.
Also If all the parts of your image are 300 dpi, it's best to have your canvas as 300, rather than stretch them out to fit say a 600 canvas, and then resize back down for the 300dpi for the site.
I always scan my raw image at 600, and work in a final canvas of 300 for the site.