I discover why don't get the image as cover.
This is the code of the image created by writer2epub:
<div><img width="51%;" alt="" src="../images/img0001.jpg" /></div>
I removed the text red marked and the image is now the cover
<div class="sgc-5">
<img src="../images/img0001.jpg" />
</div>
I hope my feedback can help you. I'm sorry if I'm being such a pain!