Pixelmator, which announced Pixelmator Pro is coming later this year, has released an interim update to the current version of its image editor that adds full compatibility with macOS High Sierra.
In addition to bug fixes, Pixelmator 3.7 supports importing HEIF image files. Pixelmator can be opened directly from Apple Photos now too. The feature, which was added to Apple Photos as part of High Sierra, allows users to choose an image in Apple Photos, but edit it in Pixelmator. All edits made in Pixelmator will be saved back to the original file in Apple Photos. Pixelmator posted a video that explains how the feature works:
Pixelmator is available on the Mac App Store.