Weird, this "prefix" thing. I'm not sure mobigen is appropriate. Is that what they're calling kindlegen these days?
OK...regarding Amazon publishing: I'm probably showing my age, but a whole two years ago, we were advised to create a cover that was 800 px max and 130kb max for publishing on Amazon. As I understood it, no matter the size cover you uploaded, their kindlegen-in-disguise resized it to under those dimensions.
Since I create my mobi file before uploading, I would resize the cover, adjust compression to just under 130, then generate my file using a combination of MPC and kindlegen.
I never worried about it. Seemed my covers were looking pretty snazzy at that size...perfectly adequate for electronic viewing...tho not so good for printing. Seemed a good compromise since I didn't really want people printing my covers anyway.
I see now, however, that Amazon is recommending 2500px max! They make this big thing about making your covers look as good as possible and obviously, bigger is better, right? Not sure about that, but being curious, I tried changing a cover in an existing epub file to a large png and converting with Kindlegen. This gave me a nice sharp cover and a HUGE mobi file. (8M)
(Lest the above sounds strange to some...I've changed my process. I now do all my cleanup in epub/sigil, then convert using kindlegen. This has given me great results, but it does require a pretty sound knowledge of epub to insert all the necessary thumbnails and TOC/beginning pointers kindle requires!)
Anyway, the last time I added a cover to a file using Calibre, it was still resizing the cover to under 800 px. I sort of trust Calibre to stay up on all this stuff, but now...I'm wondering.
What I'm asking of all you lovely folk who stay up to speed on all this is: What's the current thinking? What's the optimal size for a cover image for ebooks in general? I can evidently slip whatever I want into a file on my computer, but does anyone know what's going to happen if I publish with that large cover image on Amazon or B&N?
And as a side note on this entire topic: is Amazon still charging an extra download fee for files over a certain size? It that why they're suggesting these huge cover images (not to mention packing a million formats into a single mobi file?) (I looked on the publishing site but couldn't find that particular topic addressed anywhere)