I am just curious. We have seen images of the Milky Way Galaxy. But how do we have those images ? We Are located in the Milky Way Galaxy and we certainly don't have any thing outside the Galaxy to take pictures and send those images back to us. How do we know it looks like the way it does in the images of the entire Galaxy that we see.
We don't know for sure, but we can make extrapolations from what we can see. Galactic arms are defined by nebulae and star-formation regions as well as concentrations of interstellar gas, dust, and young, bright stars. So by measuring the distances to those things that we can see, we can get an approximate sense of how they're laid out within the galaxy, where they're concentrated and where they're not. We can't directly observe anything on the far side of the galaxy because the central bulge is in the way, but we can make best-guess extrapolations based on what we can observe of the galaxy's structure, and by comparison with what we observe of other galaxies' structures. Those other galaxies give us evidence of how galaxies form and what their dynamics are, providing a set of universal rules. And by plugging what we can observe directly about our galaxy into those universal rules, we can extrapolate things about the galaxy's overall shape. It's still a best guess, but it's an educated guess.
Here's a Galaxy Map site
that talks some more about the process behind the creation of such maps, and some of the uncertainties that remain. Here's an older page
that offers a previous, now-outdated best-estimate map, but scroll down to the second section and it contains a picture showing what we're actually able to observe directly and how those observations let us extrapolate the shape of the arms.