Why resizing is usually the first fix for a heavy image
Modern cameras produce far more pixels than most uses need. A 12 MP phone photo is about 4000×3000 px; a 48 MP sensor gives roughly 8000×6000. A laptop screen shows around 1920×1080, a blog column is often 800 to 1200 px wide, and an email preview is smaller still.
File size grows with the number of pixels, not with the width alone. Halving both dimensions leaves a quarter of the pixels, so a 4 MB JPG at 4000×3000 typically lands near 1 MB at 2000×1500 with no visible difference on a normal screen. That is why resizing usually beats any compression setting when a form says "file too large" or a page loads slowly.
Common reasons to resize: upload limits on job portals and government forms, website images where a 4000 px photo sits in an 800 px slot, email attachment limits, and templates that require an exact size such as a 1920×1080 slide background.
How the size you enter is interpreted
There are two modes, and the aspect ratio lock changes how pixel values behave.
Pixels with the lock on
With both width and height filled in, the image is scaled to fit inside that box while keeping its proportions. The result touches one edge of the box and is smaller on the other side.
- A 4000×3000 landscape photo with the Full HD 1920×1080 preset becomes 1440×1080, because 1080 px is the limiting side.
- A 3000×4000 portrait photo with the same preset becomes 810×1080.
- A 4:3 photo with the Instagram square 1080×1080 preset becomes 1080×810, not a square.
If you only care about one dimension, clear the other field. Width 1200 with an empty height gives exactly 1200 px wide and whatever height keeps the proportions.
Pixels with the lock off
The image is stretched to exactly the width and height you type. A 4:3 photo forced to 1080×1080 will look squashed horizontally. Use this only when proportions already match or slight distortion does not matter. If you need an exact shape without distortion, crop the image to the target ratio first, then resize.
Percent
A slider from 1% to 200% scales both sides by the same factor. 50% turns 4000×3000 into 2000×1500.
All presets switch the lock on, so they always behave as "fit inside".
What happens to quality and format
Resizing runs in your browser using the Canvas API, in a background worker when the browser supports it so the page stays responsive during big batches. The output keeps the input format:
- JPG is re-encoded at quality 0.92. This is high enough that recompression artifacts are not visible in normal viewing.
- WebP is also encoded at quality 0.92 and keeps transparency.
- PNG stays lossless and keeps its alpha channel. PNG files of photos remain large even after resizing; for those, consider converting to JPG.
Because the image is redrawn and encoded from scratch, embedded metadata such as EXIF camera details and GPS location is not carried over into the result. That is a side benefit when you are publishing photos taken on a phone.
Upscaling past 100% is allowed but cannot invent detail. Going from 800 to 1600 px gives a softer image with the same information spread over more pixels. A moderate enlargement of 120 to 150% is usually acceptable for screen use; beyond that, edges look blurred.
Practical targets and tips
Some sizes that work well in practice:
- Website content image: 1200 to 1600 px on the long side covers most layouts, including high-density screens at moderate widths.
- Full-screen background or slide: 1920×1080, or 3840×2160 for 4K displays.
- Instagram: 1080×1080 for square posts, 1080×1350 for portrait posts and 1080×1920 for Stories; all three are presets.
- Email attachment or messenger: 1600 px on the long side keeps photos sharp while staying well under 1 MB in most cases.
A few tips:
- Resize first, compress second. If the result is still too heavy, run it through JPG compression with a lower quality setting; the two steps together give the smallest files.
- Always work from the original. Resizing an already reduced copy upward again loses detail you cannot get back.
- Batches use one setting for every file. To give a mixed batch of landscape and portrait shots the same long side, enter the same number in both fields with the lock on: at 1600×1600, every image ends up 1600 px on its longest side. Note that images already smaller than the box are enlarged to fit it.
- Files up to 50 MB each are accepted, and nothing is uploaded: the images never leave your device.