Check out these free methods to convert WebP images that many websites use to JPG, PNG, or other formats on your Mac.
How to convert WebP images to JPG or PNG and vice versa on Mac
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How to make your iPhone shoot in JPEG instead of HEIF
Learn how to change the iPhone or iPad camera settings to make it shoot pictures in the universally compatible JPEG format instead of HEIF/HEIC.
Pixelmator now works with HEIF images & integrates with Photos on macOS High Sierra
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.
How and when to choose between HEIF/HEVC & JPEG/H.264 media formats
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. This post tells you more about it.