This past summer at WWDC, Apple introduced a new (to iOS and Mac OS) compression format for images named HEIF. HEIF is pretty neat because it allows for better compression compared to JPEG, without sacrificing quality. It's got some other fun properties as well, but it's not relevant to this post.
If you have an iPhone with an A10 Fusion processor or later (iPhone 7 and 8), you can turn on support for taking pictures in this format via the Settings app. iOS 11 also obviously adds support for viewing these files and includes APIs for developers which can write new images in that format.
Mac OS 10.13 High Sierra includes support for decoding and viewing HEIF images. There are no OS supplied libraries for writing or converting images to the HEIF format.
And because of this, Acorn currently only allows reading for HEIF files, not writing.
If you look back at WWDC videos and remember looking at early SDK headers from the 10.13 seeds, you'll see that support for HEIF looked like it was coming to Mac OS. I don't know what happened, but the decision to ship it was pulled at some point. Hopefully support for HEIF encoding will be included in a future OS update. I'll be disappointed if it isn't at any rate, as it seems really strange to me that iOS would get it while the Mac wouldn't.