@Henry : Do you know what is edge-logic? That is a technique very helpful ;)
Here, you can look at the 7 from C25. If it starts in row R1, that induces seven black cells in C24 (impossible). If it starts in R2, also seven cells (impossible). And so on, you can place some crosses and move the 7 down until R17. The 7 can thus begin in R18 or R19. The rest comes easy :)
I hope I was clear enough.

Merci à tous :)
Thanks to everyone :)

@Hatchy : En effet ;)