GIMP Version: 2.8.14
Operating System: Linux
OS Version: Xubuntu 15.04 Vivid
GIMP Experience: Intermediate Level

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When buying digital photos for the ebook covers I make, I see 72 DPI (usually for the smaller sizes) and 300 DPI photos. I know DPI means Dots Per Inch, but really what does it signify? Are the same size photos bigger files depending if they're 72 or 300 DPI? For instance a photo of 1800X2700, would the 300 DPI file be much bigger than the 72 DPI file? Are there any limitations in using GIMP's features using one or the other? Thanks.

For the average digital screen 72 DPI is fine, for print & retina display go with 300 DPI.
BTW, a 72 DPI screen is arbitrary, Why 72 dpi?

Thanks Wallace. And that's a great link. Especially since it's so current.

Thanks Odinbc. And I've heard the 300 DPI is necessary for actual book covers. In a way it's a moot point because almost all the images I see big enough for ebook covers are 300 DPI.

Consider the purpose of the image.

You want to physically print it, on paper, as a book cover
Then a typical paper back, 4" x 7 " @ 300 ppi = an image size 1200 x 2100 pixels.

Otherwise, the print resolution 300 (or anything else) is irrelevant.

But this is an ebook, going to be read on a reader/tablet/computer.

What display sizes are current, the occasional/older 7" tablet 600 x 1024 pixels.
Then a bunch of tablets 7"/8"/10.something" all 800 x 1280 px. The formfactor for these is called candybar with a ratio 1: 1.5 (The 80 px at the bottom is for controls)
As mentioned earlier some retinex displays about 1500 x 2000 and some really for video 1920x1080 HD.

This is a screenshot of a 7" tablet. 8" / 10" all going to be the same 800 x 1200 screen pixels. A laptop even worse, most have a screen depth of 768 px.



Looks to me that at the moment an image size is 800 x 1200 is good. Perhaps future proof by upping the image size to 1600 x 2400 pixels. The reader application, epub/mobi/pdf will scale to suit anyway.

Want to use 900 dpi as a setting (one of the contradictory offerings in the above reference) go ahead. Makes no difference.

Thanks Rich. Interesting. From my experience the ebook distributors (esp. Apple) keep wanting bigger and bigger covers. It used to be they wanted 400X600. Then they wanted 800X1200. Some places now want 1600X2400. The safe bet (for now) is to give them 1800X2700. Everybody can reduce them to whatever size they want.