In macOS High Sierra, it's possible to open a photo in a third-party app right from within the Photos Library using the "Edit With..." menu option, after which Pixelmator should appear following the update. Subsequently, any edits made within Pixelmator will be automatically saved back to the Photos library.
Added support for the new HEIF image standard includes the ability to import photos from iPhone 7, iPhone 8, and iPhone X, which save in the more efficient compression format by default. You can find out more about HEIF here.
Other improvements in the 3.7 update to Pixelmator include enhanced support for Photoshop PSD files, an improved Repair Tool, and a return of the ability to drag image assets from Photos and Safari directly into Pixelmator.
Pixelmator is available to buy on the Mac App Store for $30.