Allow customization of ‘maximum container width’ by percentages

[Jumballi]

When setting to my preferred display setting that’ll resize an image to the size of the width, at 100% its too large on the vertical side. Now while I can set it by pixels, but it would be too much of an effort for someone that uses multiple different devices, all with different resolutions.

So instead of skewing it by pixel size, the option to do it by percentage would be slightly more relaxing on an idiot such as myself.