Thank you vey much KevinH and DNSB for your attention,
I forgot to say I tried various svg format with inkscape before writing this post. Actually rhe svg in the epub is plain 1.0 svg format. I think this actual svg format is correct because we can see the svg content.
I also tried various XHTML tags, starting with <img/> in the past.
I also tried changing .epub to .kepub.epub (.kepub alone is not seen by kobo) before posting.
BUT, you were also both right
Actually, having "ebook3.epub" AND a copy renamed "ebook3.kepub.epub" on the same device, Kobo didn't see the second as a "KOBO epub" but as a simple epub despite its name. Deleting the first one "forced" the device to see it as a KOBO epub.
Secondly, my svg was 293px width. I didn't care since it's vector graphics and should scale fine whatever the original size.
Except that scale doesn't seem to work at all.
I made a copy of this SVG, this time scaled it up to 12000x10000px (same file size of course) and only kept minimum working tags in XHTML:
Code:
<img alt="basicdrawing" width="100%" src="../Images/basicdrawing.svg"/>
<img alt="basicdrawing" src="../Images/basicdrawing.svg"/>
And now both are displayed identically, width 100% on Kobo, while Sigil preview has the second one at the right size, only displaying the upper left corner of it.
Of course, with these tags, I tried switching the filename to the previous one, and it didn't work, displaying small images. Width=100% is simply not working on Kobo.
I guess I'd need to know the minimum acceptable width by Kobo to make svg illustrations working in Sigil preview and Kobo.
Thank you for pushing me into re-trying all those parameters
Cheers from France