With the domination of the phablet, smartphone screen sizes continue to get bigger and bigger. The 16:9 aspect ratio remains king and is here to stay, but the pixel count grows and grows:
- 1136px by 640px (iPhone 5S)
- 1280px by 720px (Samsung S3)
- 1920px by 1080px (Samsung S5)
- 2560px by 1440px (Samsung Note 4 – rumoured)
This blog displays images up to 600 pixels in width. That is only half the width of the newest phones in portrait mode; less than a quarter of the width in landscape mode. The only way to avoid ugly pixellation is to use larger imagery that can scale down for the desktop and be used in its original form for mobile.
That leaves you in what would once seemed to be a bizarre position of increasing your image sizes so that they look good on the smallest screens.
On cycleireland.ie, I use 1200px wide images, which are reasonably future-proofed (at least for the next couple of years).
You should look at that as a minimum size for your images, depending on where they are most likely to be used and how central they are to your content.