Someone answered me some days ago but it seems his/her comment has been deleted. So if anyone else encountered the same issue, I share here the solution provided : adding to <v:fill> the propriety color="none". And the undesired background color disappears!
I recommand this detailed this article to understand and correct that issue :
https://webdesign.tutsplus.com/how-to-fix-outlook-dark-mode-problems--cms-37718t
Started a new discussion: Background color for VML background images with transparency
Started a new discussion: Scaling issue on Outlook 2019 120 DPI with images and VML
The white lines issue appears mainly because outlook convert sizes in pixels into points, which can be random and create those white lines. I recommand you this article to understand this issue: https://www.actionrocket.co/email-design-review/white-lines-in-outlook