Kinja has always had a place for 640px images. This was previously the second largest size when we used 970px lede images, but has since become our largest image transform size.
While most of you won't notice a difference, those who want pixel-perfect sharpness should upload at 636px wide. This is useful for images of text, diagrams, charts, and similar images which have much to lose from a little softness. For other images such as photos, we recommend uploading at the highest resolution possible.
The lede image of this post is uploaded at 636px wide, the following image is 640px. You'll notice a slight drop in sharpness below, which is only very apparent in images with text or thin lines.