Example page: w:c:Relife:Report 203. In the portable infobox, I'm using gallery to display three images of the comparable aspect ratio.
When viewed in mobile on a narrow screen, the thumbnail of the middle image ("Title") retains its aspect ratio while the first and third thumbnail images gets cropped on the top and bottom. On wider screens/windows all three thumbnail images are cropped, but the middle one is cropped less than the others so still come out taller.
My desired behavior is for all three images to show the same height in the infobox so the infobox doesn't change height when moving between images. Forcing them to be the same aspect ratio was my attempt at solving the issue for the desktop experience, but that solution was insufficient for mercury.
After investigation, it looks like for mercury, thumbnail generated for the infoboxes are cropped against their native height, even though the overall "thumbnail" image is stretched proportionally to fit the screen/window width. This makes smaller images in infoboxes look extremely awkward on mobile.
My Samsung Galaxy S7 Edge gives me 412px width in portrait mode and 732px width in landscape mode. If I start with a 250x250 square image, it's going stretch into 412x412 then crop into 412x250 in portrait, and in landscape it'll stretch into 732x732 then crop into 732x250.
This would suggest that the only way to make portable infobox image thumbnails look good (or at least semi-consistent) in mercury across a variety of mobile devices with different screen resolutions and orientations is to start the images with large amount of horizontal pixels (perhaps 1024px width to be safe?). That might mean I have to artificially stretch my images more than 3x above their original resolutions before uploading them to the wiki. Not really a elegant solution, and the image I upload would be quite pixelated, so I'd prefer not having to do things this way.
Alternately if a way to specify or hardcode max-width for mercury can be introduced and/or cap the width against the image's native width, and just have mercury center the image if screen/window is wider than the max-width, and do not crop against the image's native height, that should also allow contributers to optimize the images for a relatively consistent experience across devices of different resolutions.