You can click any of these three to see the full sized image or download them.
1. This is the original it is 2362 X 3152 @ 300dpi 4 meg File Size saved with minimum compression.
2. This is the original with it's dpi changed so it is 2362 X 3152 @ 72dpi 4 meg File Size, it too was saved with minimum compression.
3. This is the original 2362 X 3152 @ 300dpi saved with maximum compression, file size is now 1.3 megs.
So as you can see changing only the dpi had no effect on image quality, but even with the the dpi left at 300 setting the compression to maximum wrecks havoc on the image quality as seen in the last image #3, looking at the full sized version it gets even worse than the preview.
Reason for this is, in order to make the file smaller, which is the whole purpose for compression anyway, it throws out image pixel data in order to save space and make the saved file smaller but the result if set too high is #3.
Any online image host that "optimizes" your images during upload, this is what they do to them, many popular image hosts do this, some don't even bother to tell you, others do or let you choose, also any software even Photoshop that has a "save for the web" or "optimize for the web" as an option, beware it will really compress your file to make it as small as possible with little or no regard for quality.
And you ignore the DPI setting completely unless your going to print the file as that is a print only thing, tells the printer how far apart to space the dots, that's all it does, which is why we require a set DPI for cover uploads, they are intended to be printed so the physical size matters, therefore dpi matters in that regard, but not in quality.
You may then be wondering since 1 and 2 above are basically the exact same image with only the dpi setting changed what exactly does this mean, well here you go.
Since DPI in Photoshop is referenced as Resolution which is actually short for "Print Resolution" nothing more, and yes they expect you to know this, in fact that word right there is where most of this whole confusion come from, anyway here it is, if you printed them each as is (raw) meaning no software buffer between the image and the printer to scale the image then the first one would print out at 10.5in X 7.8in due to the dot spacing being 300 dpi you can actually do the math an figure that out, and the second would print out at 43.7in X 32.8in due to the dots being spaced at 72dpi or farther apart by almost 4X, make sense.
And folks that would be the only difference now since your screen can only display 72 or 96 dpi for a PC or 72 dpi for a MAC they will both look exactly the same on your monitor, but that is because they are exactly the same as far as pixel data is concerned... ie the same number of pixels, the same color information per pixel, the same compression setting, the same format, etc.
