The category menu and sub-collection list will, by default, use the product’s first image.
To showcase a custom image for collections, please create a metafield following the steps outlined below.

Step 1 :

 From your shopify store admin > click "Settings" > Metafields and metaobjects > collections > Add definition

Step 2 : Fill in the details as shown in the screenshot below.

Namespace and Key : custom.custom_image (must)

Step 3 : Navigate to the metafield section at the bottom of the collection page. ( from store admin)

Step 4 : Click Custom image field to select image and save page.