Adjust image in qgis2web to pop up

As suggested, you can add a simple css rule to determine max width for the images.
The answer here won't work for you anymore since leaflet has changed the name of the popup content class.
There is also the problem where qgis2web uses a table inside its popups.

what you need to do, is open the index.html file and edit the ` tag.

instead of just:

<style>
  #map {
            width: 894px;
            height: 813px;
        }
</style>

add :

<style>
#map {
     width: 894px;
     height: 813px;
   }
 .leaflet-popup-content > table img {
    width: 300px;
   }
 .leaflet-popup-content > img { /*for other leaflet maps you can add rule as well*/
    width: 300px;
   }
</style>

you can change the number of pixels to match how you want you images to look.