Remove alt text from circles on homepage

Hello, i’ve been trying everything to remove the alt text which displays when you hover over the circles on the homepage. I’ve been trying things in the content-image.php and content-home.php, I feel like i’m close but just can’t quite work out the code.

Using the firebug I see that these parts are referred to as ‘post-thumbnail’.

Does anyone have any idea how to to remove this? Adding alt text in the ‘featured image’ for the post doesnt do anything either

Thanks!