Prevent picture resend on event update, handle duplicate pictures

properly in backend and add a proper default picture

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

js/src/types/picture.model.ts

@@ -1,4 +1,5 @@
 export interface IPicture {
+  id: string;
   url: string;
   name: string;
   alt: string;