Passport photos rarely get rejected for dramatic reasons. It's almost always one of a handful of small, fixable things. Here's the full list, worst offenders first, with how to avoid each.
Wrong size or head proportion
The single most common cause. Each country specifies both the photo size and how big your head must be within it — the US wants 25–35mm chin to crown on a 2×2 inch photo; India wants a notably larger face. A generic square crop won't match. The passport photo maker sizes the head to the exact band per country, which removes this entirely.
Wrong background colour
White is not universal. The UK rejects plain white and wants light grey or cream; for Schengen visas, Switzerland requires a grey background and rejects white (light grey is the safe choice everywhere). Using the wrong shade is a frequent bounce — so the background should be set per country, not assumed.
Glasses, smiling and expression
Most countries no longer allow glasses at all. A smile, an open mouth or raised eyebrows will also fail a biometric check — keep a relaxed, neutral expression with your mouth closed.
Shadows and uneven lighting
Shadows on your face or on the wall behind you break the "plain, uniform background" rule. Soft, even light from a window and a small gap between you and the wall fixes most of it.
File too large (or too small) for upload
Online portals cap the file size — Indian exam and government forms often want 20–50 KB, and other portals have their own limits. A phone photo is far too big. Compress the finished photo with resize to 20 KB or 50 KB to fit.
Old photo or wrong document type
Most countries want a photo taken within the last six months. And a visa photo isn't always the same as a passport photo — check the right spec with the visa photo maker if you're applying for a visa.
Get these six right and rejection becomes very unlikely. The fastest way is to let the tool handle size, background and the compliance check, then compress to your portal's limit.