How to Add a Custom Size for Thumbnails for WP - Gallery

Question

I am using "Lightbox Plus" plugin to create lightbox image overlay on the top of thumbnail galley. Now my question is how I can set a size for Thumbnails without affecting on the lightbox image overlays. I mean when I try to set a scale for Thumbnails through wp-Gallery edit option it apply the size even to the overlays (I would like to keep the overlay size as big as possible but this function makes them small like Thumbnails size) I also see this line of code in codex:

 `get_the_post_thumbnail($id, array(100,100) ); // Other resolutions`

but I don't know how and where to use it.

Thanks

Answer 1

Use add_image_size():

add_image_size( 'custom-name', 123, 456, true );

...will create image size custom-name, with dimensions of 123x456px, hard-cropped.

Edit

Re this comment:

Thanks for comment but honestly I got more confused!can you please let me know what is the 'custom-name'? How I can associate this name with Gallery thumbnails?

The custom-name is arbitrary. It's whatever you want it to be. You could call it gallery-thumb, if you want to:

add_image_size( 'gallery-thumb', 123, 456, true );

Then, you would output that image size by passing gallery-thumb to the_post_thumbnail():

the_post_thumbnail( 'gallery-thumb' );

In your case, I assume you're using the core WordPress [gallery] shortcode? If so, you can pass the size attribute:

[gallery size="gallery-thumb"]