[Liudprand]

Since Calibre doesn't seem to have the native capacity to compress png files, but just hangs for ages trying to do so but failing, I routinely convert png files to jpg, using ImageMagick - usually (maybe 80% of the time) without a problem, using the following steps:

1. Export png image(s) to a separate folder.
2. Change file extension within ebook for those images to jpg.
3. Delete the images from the ebook.
4. Convert exported png files to jpg.
5. Import new jpg images back into the ebook.

I often have trouble, though, with some png files even after this procedure, which as far as I can work out should do the job. The images concerned don't show up properly in the book - just empty boxes where they should be.

Is there a step I'm missing out?

Thanks in advance!