Thank you very much for your help, really.

I can now crop it as I want, but when it is displayed on the profile, it is still square and not the cropped image in a rectangle.Could you tell me why?

By default, Elgg config has these sizes for avatars:

'icon_sizes' => [
   'topbar' => ['w' => 16, 'h' => 16, 'square' => true, 'upscale' => true],
   'tiny' => ['w' => 25, 'h' => 25, 'square' => true, 'upscale' => true],
   'small' => ['w' => 40, 'h' => 40, 'square' => true, 'upscale' => true],
   'medium' => ['w' => 100, 'h' => 100, 'square' => true, 'upscale' => true],
   'large' => ['w' => 200, 'h' => 200, 'square' => true, 'upscale' => true],
   'master' => ['w' => 10240, 'h' => 10240, 'square' => false, 'upscale' => false, 'crop' => false],
],

You can override this config via custom plugin.

Use My plugin again.

1 - Open elgg-plugin.php and add this code before last ];

'events' => [
    'entity:icon:sizes' => [
        'group' => [
           \MyPlugin\SetIconSizes::class => [],
        ],
   
        'user' => [
          \MyPlugin\SetIconSizes::class => [],
        ],
      ],
],