ikaring

Hi franck_b.
Just set blog id of your main site instead of get_current_blog_id().
Something like <?php echo get_home_url( 1 ); ?>, when main site id is 1.
If that is not what you need, you’d better set new thread for your case.

Since correct value is stored when echo $taxid;, it should be returned as is.

There is a space after $taxid. Isn’t that the cause?
return $taxid ;

And what is the returned value? NULL?

In Codex, it saids as below, though [“term_taxonomy_id”] seems to be integer.

Warning: string vs integer confusion! Field values, including term_id are returned in string format. Before further use, typecast numeric values to actual integers, otherwise WordPress will mix up term_ids and slugs which happen to have only numeric characters!

Try
var_dump( intval($taxid) );
or change $taxid to
$taxid = $term->term_id;

Just for checking: the 3rd arg of get_term_by, ‘someterm’, is a taxonomy name.
eg.
‘category’
‘post_tag’
‘your_custom_taxonomy’

Do you see class attr and srcset attr in the img tag?
If not, you might have changed the output of the_post_thumbnail with filter hook or some sort.
If yes, I would check if there is a syntax error in functions.php before/after the add_image_size.

BTW, what if you call the_post_thumbnail(); without args?
Does it show thumbnail image?

I dont know if it is possible to crop images as you described.
But it might be possible with css, no hard crop image.

Suppose you have markup like:
<div class="container"><img src="yourimage.png" class="fitimage"></div>

Then css may look like:

.container {
  width: 300px;
  height: 300px;
  display: table-cell;
  vertical-align: middle;
  text-align: center;
  background-color: #fff;
}
.fitimage {
  width: auto;
  height: auto;
  max-width: 100%;
  max-height: 100%;
}

Hi. Is it the pink circle?
Have you tried .site-branding { text-align: center; } ?

What is the value of $attachment_id?
Is it set beforehand?