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If you want to host the images on microcosm, then add them as attachments, then edit the post to have them embedded and remove them as attachments.
Usually it's not relevant for me if an image is hosted on microcosm or anywhere else,
but, while your option sounds sort-of strange at first it actually works very well, and is less faff than uploading to some other image host beforehand.
Cheers!
If you want to host the images on microcosm, then add them as attachments, then edit the post to have them embedded and remove them as attachments. Works for me.