<div class='rel-fill image-link'>
<% category = Category.where(id: article.category_id).to_a[0] %>
  <%= link_to category_path(id: category.id) do %>
		<div class="category-label">
			<%= category.title %>
		</div>
	<% end %>
	<%= link_to article_path(id: article.id ) do %>
		<%= image_tag article.image.url(:large), class: "#{image_class}" %>
	<div class="title-box">
		<%= article.title %>
	</div>
	<% end %>
</div>	
			
<div class='rel-fill image-link'>
<% category = Category.where(id: article.category_id).to_a[0] %>
  <%= link_to category_path(id: category.id) do %>
		<div class="category-label">
			<%= category.title %>
		</div>
	<% end %>
	<%= link_to article_path(id: article.id ) do %>
		<%= image_tag article.image.url(:medium), class: "#{image_class}" %>
	<div class="title-box">
		<%= article.title %>
	</div>
	<% end %>
</div>	
			
Set img_link image to medium
Set img_link image to medium
