Resizing changes pixels; compression changes bytes
These are two different levers and it helps to keep them apart. Resizing changes how many pixels the image has — a 4000 px wide photo becomes 1200 px wide, and the other 2800 columns of pixels are gone. Compression keeps the pixel count and stores those pixels less precisely, which is what the quality slider controls.
When a file is too large, resizing is almost always the bigger win, because file size scales with area: halving both dimensions leaves a quarter of the pixels. Reach for quality second, to fine-tune.
If the dimensions are already right and only the file size is wrong, use the JPG compressor or the PNG compressor instead.
Why the aspect-ratio lock matters
Aspect ratio is the relationship between width and height. Change one without the other and the image stretches — faces widen, circles become ovals. The lock keeps that from happening by calculating the second dimension for you.
Turn it off only when you genuinely want a different shape and you accept the distortion. If you need a different shape without distortion, you want to crop the image instead: cropping discards the parts outside the new frame rather than squeezing everything into it.
Common target sizes
Platforms change their specifications regularly, so confirm against the destination. These are the sizes people ask for most often.
Frequently used image dimensions
| Use |
Pixels |
Ratio |
| Open Graph / social share image |
1200 × 630 |
1.91:1 |
| Blog header |
1600 × 900 |
16:9 |
| Instagram square post |
1080 × 1080 |
1:1 |
| Instagram story / Reel |
1080 × 1920 |
9:16 |
| YouTube thumbnail |
1280 × 720 |
16:9 |
| Profile avatar |
400 × 400 |
1:1 |
| Full-width hero on a 2× display |
2560 × 1440 |
16:9 |
Resizing without losing quality
Making an image smaller always discards pixels, in every tool — that part is unavoidable. What you can control is how much damage the process adds on top.
- Always start from the largest original you have. Resizing a copy that was already shrunk compounds the loss.
- Resize once. Every JPEG save re-compresses the image; repeated round trips visibly degrade edges and flat areas.
- Do not enlarge. Scaling up cannot invent detail — it interpolates, and the result looks soft.
- Keep quality at 80% or above for photographs. Below about 70% you start to see blocking in skies and gradients.
- Watch out with transparency. The output here is JPEG, which has no alpha channel, so transparent areas of a PNG or GIF will not survive.