If you own an iPhone 7 and newer or a 2017 iPad Pro, you may have noticed that updating to iOS 11 defaults the Camera app to capturing photos in Apple's new High Efficiency Image File Format (.HEIC files) and videos in the High Efficiency Video Coding (HEVC or H.265) format instead of the ubiquitous JPEG and H.264 media formats.
Pixelmator for Mac was recently updated on Mac App Store with full macOS High Sierra compatibility, including the ability to open images saved in Apple's new efficient HEIF format and integrate with the stock Photos app as an external image editor.
iOS 11 introduced support for modern HEIF and HEVC codecs developed by the Moving Picture Experts Group (not Apple's proprietary formats) that keep high-resolution photos and videos taken with iPhone and iPad from eating up all your available storage.