The image size guide

An image's dimensions are its width and height in pixels — for example, 1920×1080 means 1920 pixels wide and 1080 pixels tall. Dimensions are the single biggest factor in file size: doubling both width and height quadruples the pixel count, which roughly quadruples the file size at the same compression level.

For the web, 1920px wide is a sensible upper bound. Most desktop displays render at 1920px or less for the main content area, and even high-DPI displays only require double-density (so 3840px sources for a 1920px render). For social platforms, dimensions are dictated by the platform itself — Instagram renders at 1080px wide, YouTube thumbnails at 1280×720, Twitter cards at 1200×675. Going larger than the platform's display size is wasted bandwidth: the platform downscales on upload.

Going smaller than the display size is worse — the platform upscales, which softens edges and creates a visible quality loss. The safe rule: match the platform's expected dimensions exactly, or slightly above for retina rendering.

The aspect ratio is the proportional relationship between width and height. A 1920×1080 image has a 16:9 ratio (1920 divided by 1080 = 1.78); a 1080×1080 image is 1:1; a 1200×627 share card is 1.91:1. Aspect ratio matters because platforms crop or letterbox anything that doesn't match what their UI expects.

For example, Instagram square posts crop to 1:1. A 4:5 portrait photo uploaded into a square slot gets its top and bottom shaved off. The 'safe area' for important content is always the centre — but starting from the correct aspect ratio in the first place avoids the loss entirely.

The most common aspect ratios on the modern web:

Three factors determine an image's file size in bytes: pixel dimensions, colour depth, and compression. You can't change colour depth without visible posterisation, so realistic optimisation focuses on the other two.

Dimensions: halving width and height reduces the file size roughly 4×. The biggest single win on most websites is realising that hero images uploaded straight from a phone or camera are typically 4000+ pixels wide while rendering at 1200px — three quarters of every file is wasted bandwidth.

Compression: JPEG at quality 85 produces a file that's visually identical to quality 100 for most photographic content, at half the size. WebP at quality 80 typically beats JPEG q85 by 25–35% with no visible difference. The compression slider is the single tunable that gives the most control over file size.

Target sizes most platforms enforce: YouTube thumbnails ≤ 2 MB. Email attachments ≤ 1 MB (Gmail compresses anything over that into a Drive link). Many forms cap at 500 KB or 200 KB. The compression target presets below land any photograph reliably under each of those ceilings.

Four image formats dominate the web in 2026: JPG, PNG, WebP and AVIF. Each has a clear best use case — and the wrong choice can multiply your file sizes by 5× or more.