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Plain explanations of what actually goes wrong with images, written for people who have just been told their file is not acceptable and would like to know why.

• How to get an image under a size limit without wrecking it There are two ways to make an image smaller, and almost everyone reaches for the worse one first.
• Why websites reject your images The complete list of things that actually go wrong on upload, and how to tell which one you have hit.
• HEIC explained: why your iPhone photos won't upload Your phone saves photos in a format that is better than JPEG and that most of the internet cannot read.
• JPEG vs PNG vs WebP — which to use and when Three formats, three jobs. Choosing correctly is often worth more than any amount of fiddling with quality.
• What happens to quality when you compress Compression discards specific things in a specific order, and knowing which explains everything you see.
• What image dimensions actually mean Pixels, aspect ratios and DPI are three ways of describing related but different things.
• Why your phone photo is 8 MB Upload limits were set when a good photo was 500 KB. Phones have moved on; the forms have not.
• How to fix a sideways photo It looks fine on your phone and sideways in the email. The rotation is stored as a note, not in the picture.