On the plugin settings page (/wp-admin/admin.php?page=gallery-plugin-pro.php) you can set the size of thumbnails for certain images ("Image size for thumbnails" option) and album covers ("Image size for the album cover" option).
Below you can select "Crop position" in horizontal and vertical directions.
We would like to explain the way this function works based on the example:
Let's assume you set "Image size for thumbnails" (the thumbnail of a regular gallery image - not an album cover) to 160 px (Width) and 120px (Height). The "Crop position" is set to Horizontal: center - Vertical: center.
Then, you have created a gallery and uploaded several images to it (for instance, 3 images).
The plugin will create the thumbnails the following way:
1. The first image with a size of 1600 to 1200 px will not be cropped at all, since when reducing it to the "closest" value in "Image size for thumbnails" option the plugin will get an image with a size of 160 to 120 px, and there will be nothing left to crop.
2. The plugin will reduce the second image (2000 to 1200 px) by 10 times. Then it will get the intermediate image with a size of 200 to 120 px, so it will have to crop the "extra" 40 px (40 = 200 - 160) on the right and left, depending on the settings specified in "Crop position". Since we have set the center value for the horizontal direction, the plugin will "cut off" 20 px on the right and left sides of the reduced intermediate image. If we had specified Horizontal: left in "Crop position", the plugin would have "cut off" 40 px on the right etc.
3. Likewise, the third image (1600 to 1800 px) will be reduced by 10 times, which will leave us with an image size 160 to 180 px. Thus means that the "extra" 60 px need to be cropped at the top/bottom of the image (60 = 180 - 120). You need to check the settings in "Crop position" again. Let's say it is set to Vertical: center. This means that the plugin will "cut off' 30 px at the top and the bottom of an image.
The same can be applied to album covers.
Also, please note that if you have changed thumbnail size and the Crop position on the plugin settings page, the new settings will be applied to newly created images only. If you'd like these settings to be applied to the existing images as well, you need to click "Update images" button at the top of the settings page and wait for the process of generating new thumbnails to end (if there is a lot of images on your website, this process may take a while).