Hey @poultonjoe, in that case you would need to use li-object:src, so add the :src modifier. And I think you also need to specify the dimensions with the url filter. So in the end it should look something like this:
li-object:src="collection.metafields.custom.collection_image | image_url: width: 1000, height: 1000"
li-object:alt="collection.metafields.custom.collection_image.alt"
You can play a bit around with the image_url filter, here is the documentation:
https://shopify.dev/docs/api/liquid/filters#image_url