Upload the first one using the upload button, then post the rest of all in the body content, there is a image icon button for you to upload the images to the body content.

I don't know Lemmy well enough to advise on that side of things, but you can share multiple images via one link using an Imgur.com album for example. Hope that helps. :)

To save space, after uploading the first picture, you can embed all the others if you have uploaded them elsewhere, I did that for a post since I had previously created a Thingiverse page, I just took the image link and used it in the body. Just use ! [] (imagelink) without spaces and your images will show up as such in this instance.