Aha, I think I just found the problem! I was using the "Attachments" button below the reply box to add images, and this option doesn't actually allow anything besides .gif and .jpg, but adding the same file via the "Insert/edit image" button works and adds it directly to the post instead of as an attachment. Thanks for replying and getting me to look at things for more than 30 seconds!

Looks like the attachment button is redundant, with the other option being more useful and even adding the image directly to the post.