[roger64]

OK Solved. I split my file in five "chapter breaks", including one for one image.
I also suppressed the second image which was in .gif format.

One of these two things did the trick. BTW the end result is perfect. So I would recommend using this extension which works well with Sigil.
I use OpenOffice 3.2 with Linux.

So I did not have to test the Validator. I keep it for the next time.

Thanks very much for your help.