When viewing an image through the issue attachment preview, the image renderer (jira-fileviewer-plugin) applies the following CSS property:

image-rendering: pixelated;

This prevents images from being automatically scaled up when zoomed in (Mozilla Developer doc).

However, this does not occur when the image is rendered in a Wiki Style Renderer (TinyMCE) field or comment. It instead uses the default auto attribute. This typically applies bilinear resampling in most web browsers to smoothen the image.

Wiki image (thumbnail)

Wiki image (complete)

Customers with pixel-perfect requirements (especially in the design industry) want to apply the same pixelated attribute to all images. This should be implemented via a profile preference or user feature flag.