We can able to fetch the Custom Image Type product attribute value from the Product object by just fetching the product object.
Let’s say you have created a custom image_type product attribute called, brand_logo and you want to fetch the value saved for a product by product object. You can refer to the below logic to fetch the image value.
You can fetch the value of the image attribute by calling the below line,
I hope you have fetched the Product Object by the ProductRepositoryInterface, You can check the logic, Fetch Product Object By SKU
$product->getData('brand_logo');
Using the above stuff, You will get the file path of the image in the response.