On Kindle for PC I can see that images change size to suit when I make the window smaller or bigger. However I can't find any information on whether (JPEG) images within a MOBI or KA8 are scaleable for different screen sizes on eReaders? Would I be right to assume that because it does on Kindle for PC it will adjust for different sized eReaders too like Kindle Fire and DX?
From what I read for ePUB format I'd need to use Scalable Vector Graphics, so if MOBI does image rescaling anyway that would save some hassle. Been looking around and can't find any info would love to know!
I have seen a lot of sub-sized images on the Fire, but mine show up okay. Supposedly if you use 600x800 pixels as your ideal, you will fill the screen (though not vertically on the Fire, which has a skinnier layout) every time.
There are more sophisticated techniques that provide advantages in some cases, such as scaling the containing div to the screen percentage and then scaling the image width to 100% of that, but for basic full width images this should work well enough.
"From what I read for ePUB format I'd need to use Scalable Vector Graphics"
Vector graphics are cartoonish in nature. They don't work for photographs too well. You can take a photograph and convert it to a vector graphic, but it will come out looking more like a cartoon than a photo for the most part, and it still won't scale larger without breaking up. I'm oversimplifying things, but that's the gist of it. But if you start in vector format to begin with, drawing/painting a picture, then it will scale fine.
But like the previous poster said, just keep your photos at either 600 x 800 or 600 x 900, and they'll look fine on pretty much any reader or tablet.
Thanks for the input, to be honest it looks a bit complicated. With my limited knowledge here's how I've tried to work around it: Created it as html in old version of Dreamweaver (just putting the text and pics in, no programming), used Calibre to make into ePub then used the Kindlegen plug in through Kindle Previewer to make into MOBI.
I chose to do it that way because in order to get the cover in Kindlegen the instructions seem to say I'd need to program some html which I can't do, so knowing that Kindlegen also converts from ePub I used Calibre to include the cover and make into ePub and convert from that.
Many of the pictures I started with in the original html were over 2000 x 2000 but Calibre then Kindlegen compressed them anyway, and it looks absolutely fine through all devices in Kindle Previewer with the pictures resizing to suit so I think I'm home free and that it is good to send to KDP? not sure, I can't see how to find out what size the pictures are within the kindle content file but I guess I can take their own software as seal of approval?
There's no need to use Calibre, and many reasons [i]not[/i] to do so.
If you just want the cover image included in your book, I've attached an example OPF that makes this very simple, even with limited knowledge of HTML/XML. Just change the filenames within the OPF to match your files, or the names of the files themselves to match the OPF. This file assumes that you have "start" and "toc" anchors/bookmarks inserted as you would when uploading to KDP, however you can also edit those in the guide section if you are using something different.
After making the necessary changes you would simply run the OPF through KindleGen (or Kindle Previewer) as you would have previously with the HTML file.
On Kindle for PC I can see that images change size to
suit when I make the window smaller or bigger.
it is way more complex than that, ie even with same window size for KFC we see KindleGen will SCALE an image ie this is not resize but scale up to NINE different sizes to avoid a white space on a Kindle page.
here is the proof, try it yourself as Amazon changes algorithms more often than socks.