[KevinH]

That file is listed in your OPF manifest as having an image/jpg file type but it reality that file should be renamed to 1.png and all links to it need to be updated.

If you are using an epub editor like Calibre or Sigil, this would be trivial to do. If you have to do this manually, then make a copy of the epub and rename that copy to end with .zip (as an epub is a specially constructed a zip container). Then unzip that file.

Then look inside the OEBPS folder and inside that the image folder. You should be able to see 1.jpg and should rename it to end with .png and not .jpg.


Then back in the OEBPS folder find your OPF file (ends with .opf) and open that file in a text editor to find the reference to 1.jpg and change it to 1.png and replace the image/jpeg with image/png.

Finally you must search all of the xhtml/html/htm/svg files to look for '1.jpg' and replace it with '1.png'.

Or just use Calibre or Sigil.

Edit:

And I agree that using better names that 1.jpg or 1.png is a very good idea. But mass renaming files is something best left for epub editing software.