On default, Loma is displaying full image size on archive page.

You can replace this rule wih some php editing.

Add this snippet into your theme functions.

Parameter(s) :

Line #39 :  the_post_thumbnail(‘medium’);

Replace medium with your desire add image size name.

Note(s) : 

Do not forget to regenerate your thumbnails using Force Regenerate Thumbnails WP Plugin.

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