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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.

Every one of these connects to a tool that does the thing it describes. If you only need the fix, start at the tools.

• 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.
• Photos, signatures and scans for an application form Application portals impose the strangest rules on the internet, including minimum file sizes. What each one means.
• When a scanned document is too large A scan is not a photograph, and the advice that works for photographs makes text unreadable. The rules invert.
• DPI, PPI and resolution explained DPI is a number in a header, not a measure of quality. What it controls, and why “72 DPI for web” is a myth.
• Aspect ratio explained, and why platforms crop your images Getting the shape wrong rarely produces an error. It produces a crop somebody else chose for you.
• Crop, or add borders? Two ways to change an image’s shape: throw picture away, or give up frame area. When each one wins.
• What EXIF data is, and what your photos are carrying GPS coordinates, camera serial numbers, dates. Which fields identify you, and when they survive.
• Does a watermark actually protect your photos? Partly. It is a deterrent and an attribution, not a lock — and knowing which you need decides how to place one.