When I edit a post, I can write img tags for images, and set the size of the image to fit my post, and not be so big. As you can see in the screen shot, in the preview it works.
(Don’t mind the alignment of the text in the edit, I just temporarly aligned it to the left so it would be more convenient to write the img tags)
If that doesn’t fix it, then it probably has something to do with how Discourse bakes the lightboxes for larger images.
Edit: Testing on try.discourse shows that using the examples I provided when initially posting works as intended, but trying to go from what @Itzikiap is using to one of my examples by editing an existing post does not correct the baked version of the post.