It all depends on your client and how you add pictures to your post. In Amethyst it's easy.

In Nostr pictures aren't really posted. A note showing a picture really only has the address (URL) to that picture. You can put as many addresses in a note as you want, even if you have to paste them in yourself.

Most of the pics I share on Nostr are ones I uploaded to imgbb.com. When I type up a note, I paste the addresses to my uploaded pics.