Thank you for this good suggestion.

Feature already exists. Right click image and choose "Crop picture" from the context menu. Set the percentage in the section "Scale".

Both Marco and Edwin fail to locate feature -> feature poorly located.
Percentage scaling of image should be under Position and Size...
Or better yet, fields width and height should accept percentage input alongside inch, centimeter etc.

With your permission, I reopen this bug for intuition sake.

I am by no means an expert but in my mind, cropping is not the same thing as resizing.  Cropping usually means dropping parts of the image while keeping the same resolution whereas resizing allows you to keep the full image but make it smaller or bigger.

With that in mind, the feature Regina points out belongs in the Position and Size dialog box.

The "scale" feature (reached from context menu->picture->crop) rounds to 1%.  This is much too coarse for small percentages; a full decimal number would be preferable. (The up and down arrows might still work in integers.)

There is a workaround, but it's tedious and error-prone: do one's own percentage calculation for "image size" both in width and height