Manual:$wgThumbUpright

Details
This is a scaling factor applied to image thumbnails when the 'upright' parameter is used. The user's preferred thumbnail width is multiplied by this scaling factor.

This allows a nicer look for upright (portrait) images without the need to specify a hardcoded pixel width in the page markup. The scaled images' widths will be rounded to the nearest 10 pixels for cache health.

For example, assume File:Portrait.jpg has dimensions 400×800. User X's preference is to see thumbnails that are 300px wide. When User X views  , they will see a thumbnail of width (300 × $wgThumbUpright). By default that would be (300 × 0.75) = 225 pixels wide.