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Source: https://writers.stackexchange.com/a/42926
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Any online image can be scaled to 100% of the width of the webpage, that means it will always fill the available width, regardless of the actual pixel size.

## compression is more important than pixel size

**Somewhat more important than pixel size is compression.** [TinyPNG](https://tinypng.com/), [pngquant](https://pngquant.org), [ImageOptim](https://imageoptim.com/mac), and other online services can drastically reduce the file size of a large image by selectively removing the possible colors, typically reducing it to ~25% of the original download. Depending on the image, this compression can look almost "lossless", much cleaner than the chunky squares of JPG. **These tools will allow your online images to be much higher in resolution while having a lower memory footprint and faster download time.**

## What size, specifically

Screens become larger and more dense. If you want to future-proof your photos, or display them on a 4K television, or if you want users to zoom into detail, the resolution must be higher. **Use the compression tools I linked above to reduce their file size while keeping the higher resolution.**

As with Cyn's answer, Wordpress, Facebook, and other blogging software will resize your photos in the feed, allowing users to click to see the full resolution. Your blogging software, or its theme may be controlling this for you. **There is no way to say 1 particular size is the best. It depends on the kind of information in the photo, your website design, and the user device** which varies by country so it's worth checking.

## **All internet traffic is increasingly skewing to mobile devices**

Although _engagement_ (the time spent on the website) is still slightly higher on desktop, **most people will find your website on their mobile device**. See this breakdown of [mobile screen size usage by country](https://deviceatlas.com/blog/most-used-smartphone-screen-resolutions). 2 sizes are most popular: 750\*1334, 1080\*1920 (screens are oriented for portrait). At a distant 3rd place are "budget" screens at 720x1280 and the "retina"-like high-density screens that double that (1440\*2560, 1440\*2960). The high-density screens aren't for images, but to make text appear sharper.

Original score: 2