Hi Claire,
I would recommend not using images for
the text of chapter headings as it makes the book less accessible
and less amenable to the reader changing text sizes to their
preferences.
1. I don't think these images would be
particularly large as JPEGs. However, if you have repeated
identical images, such as the scene breaks, you can ensure the
same name is specified in the properties of all the identical
images, and that the option "Use specified image file names" is
checked in your configurations. This will cause only one image to
be output to the file for each unique name.
If you need the scene break images to
have transparency on Kindle, you will need to use a GIF and in
your configurations, set "Convert images to JPEG" to "All except
GIF".
2. You can set the image width to a
percentage of the screen, e.g. 90%. That should in theory be
honoured by all reader software. You can centre it by putting it
in a centred paragraph.
Regards,
Julian