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Work out where the bytes are going first

Reducing an image file is easier if you know what is making it large. In practice it is almost always one of three things: too many pixels, too much quality, or a format that is wrong for the content.

Check the pixel dimensions before anything else. If a photograph is 4000 pixels wide and will be displayed at 800, you can cut roughly 85% of the data with no visible difference at all — more than any compressor setting will find.

The levers, in order of how much they help

A sequence that works

• Crop to what you actually need — unused edges are pure cost.
• Resize to the largest size it will be displayed at, doubled for high-DPI screens.
• Choose the format for the content: WebP or AVIF for photographs, lossless WebP for screenshots and line art.
• Compress at a quality you have looked at, then check the small details at 100% zoom.
• Keep the original. Every step above is one-way.

The mistake to avoid

Converting a JPEG to PNG to "reduce size" does the opposite — PNG will be several times larger, and it cannot recover the detail the JPEG already discarded. Lossless formats only ever make a photograph bigger. Move toward lossy formats for photographs and lossless ones for graphics, never the other way around.