I had the same problem so what I've done is chage the resize.image.class.php file which generates the thumbnails.
When I set 200px width and 200px height in the module the thumbs are now generated to these 200px/200px fixed width and height but keeps the aspect ratio.
If the height for example is then bigger than 200px (because of the aspect ratio) the thumbnail only 'crops' out the needed 200px

Diff (left old/right my updated version): https://www.diffchecker.com/4GxLWgkA
quick & dirty hack - in theory this $crop true/false could be a new option in the module and so on... but don't have the time for that at the moment

my edited resize.image.class.php file (overwrite /modules/mod_j51thumbsgallery/resize.image.class.php) attached

// Find optimal resize coefficient

$resize_coef = max($this->new_width / $width, $this->new_height / $height);

// Calculate coordinate offsets

$src_x = ($width - $this->new_width / $resize_coef) / 2;

$src_y = ($height - $this->new_height / $resize_coef) / 2;

// Calculate used source image sizes

$width = $this->new_width / $resize_coef;

$height = $this->new_height / $resize_coef;



ImageCopyResampled($image_c, $new_image, 0, 0, $src_x, $src_y, $this->new_width, $this->new_height, $width, $height);