There is so much conflicting information on the web about what size your ebook cover should be - both in file size and the dimensions. I'm not a graphic designer but I have spent countless hours researching this topic and wanted to share what I have gleaned from numerous sources.
I am going to give you a one size fits all approach that I have used for my own ebook cover on Amazon, iBooks, Kobo and Goodreads. The guidelines from the e-tailers are fuzzy at best (no pun intended) so it's little wonder that no one seems to really know the best way to get your cover looking great on these sites. But using the approach I've outlined below you can see how my book cover ended up looking on Kindle, Goodreads, iBooks and Kobo.
My cover was designed in Adobe Illustrator and saved as a jpeg file for upload. Here is what I did.
1. Your file needs to have a colour profile of RGB, not CMYK and created at 72 or 150dpi. Amazon says 72dpi. I always go for 150dpi. Kobo has a crazy 900dpi recommendation. I have no idea why. I just ignored it.
2. Create your image at 1800px wide and a minimum of 2500px on the longest side (this is Amazon's requirement). The only requirement I could find for iBooks is a minimum of 1400 px wide. And Kobo doesn't give any pixel guidelines at all.
My book cover was designed at 1:1.5 ratio so, 1800px wide x 1.5 = 2700px long. The cover image is 1800px wide x 2700px long. Perfect! Amazon prefers a ratio of 1:1.6 but will accept 1:1.5 just fine. Kobo and iBooks don't stipulate ratio. They accepted my 1.5 ratio without any trouble or image distortion.
3. Save your image as a jpeg file at its original size and without any compression or optimisation. YES, you heard right. Save that sucker at the monstrous size you created it at. Resist all temptations to reduce the image percentage or size, do not tick optimisation or progression buttons, save the ORIGINAL artwork as a jpeg.
'But wait!' I hear you say, 'I've heard that Amazon charges delivery fees so I have to keep my cover file size as compressed and small as possible. Otherwise, I'll get slugged 15c per mb when someone buys and downloads my book!' WRONG. It is true that delivery fees are charged, but what you probably don't know is that Amazon optimises and compresses your jpeg once they receive it. In fact they state in their guidelines to send them an uncompressed file. So please, please, please ignore all the terrible advice on the internet to compress as many kilobytes out of your file as possible. If you do, you'll likely end up with a blurry image on your sales page and customers' e-readers. EEEEKKKKK!!!! Not a good start for your book.
4. After you've saved your jpeg, make sure your file is less than 50mb. That's the maximum file size Amazon will accept. Unless you have ridiculously intricate cover art, with a gazillion layers and effects and elements, your file should be not anywhere near that size. Mine was 1.8mb full size.
Kobo however, needs your file to be no larger than 5mb. Note that in the screenshot below, the type says 1mb but the image says 2mb. This is from their publisher's pdf guide. But when I went to upload my book cover onto Kobo, it then said maximum file size of 5mb.
And as for iBooks... soooo super helpful.....not, just says that the image has to be at least 1400px on its shortest side. My full size, uncompressed, unoptimised file loaded up great.
For your own peace of mind, here are the e-tailer's guidelines below. I hope this saves people some time when creating their ebook covers. Gabriel x