They can’t.

There are no photos from outside the galaxy.

Even knowing the precise appearance of the Milky Way is virtually impossible because we can only see it edge-on.

What astronomers have to do is to measure the distances to a very large number of stars and make a map from the direction and distance to each of them. Then you can plot that on a polar coordinate map to see where the arms must lie.

This is an extremely difficult and error-prone process!

Before about 2017, it was thought that the Milky Way had four spiral arms. But the Spitzer space telescope discovered that there are only two:

But you can see that there are kinda fragmentary sections of other arms that could easily be mistaken for entire arms.

So prior to Spitzer – this was the accepted view;

The best NASA or the astronomers can do is show you a picture of another galaxy, or a computer simulation of this galaxy.

This might be one reason that space deniers think that “space is all CGI,” or that “NASA lies,” but they don’t. The will never show you a “picture of the Milky Way from outside.” If NASA or an astronomer shows you a galaxy from the outside, they will tell you which one it is. If a science article shows a picture they got from NASA, they might have mislabeled it.

But it is not currently possible to photograph the Milky Way from the outside.