This is the only given option to images…
https://www.wix.com/corvid/reference/$w/image/fitmode
But perhaps you could be tricky and insert 2x different sizes of pictures onto your page and hide one of this 2 different sized images.
And when you want the bigger one you let appear the bigger one an disappear the smaller one or vice versa.
But because you are using “Wix Stores” (i assume this), it could be getting hard to achieve that aim.