Add id="" to thumbnails in index

Why do you need to add id? You can access it with the parent tag.
for example if the code looks like this . . .

<div id="post">
<img class="alignleft size-thumbnail wp-image-104" title="my_image" src="my_image-150x150.jpg" alt="" width="150" height="150" />
</div>

Then you can access it by #post img{width=200px;}

I need the ID tag because I’m adding custom jquery tooltips to thumbnail images.

do something like this:

$attr = array(
  'id' => 'slide_thumb_'.$post->ID
);
echo get_the_post_thumbnail( $post->ID, 'thumbnail', $attr );

Then you can use #slide_thumb_<your_post_id> as selector.

also see https://codex.www.ads-software.com/Function_Reference/get_the_post_thumbnail

BR\ME

Could you please tell me how to implement that?