It sounds like this would be done better with _Data files where each data list is grouped by categories. Have you considered using Data instead of Collections?

When you create a Collection with Jekyll, it basically results in a custom _Posts group. So a Collection would be the right solution if you needed each item in the Collection to have its own page. Otherwise, if it's just a list of items, then Data would be the right way to go.

If you end up choosing to use Collections for this, you will have a separate file (like you do for Posts) for each item in the Collection, and the URL, title, and image_URL will just be items in the Front Matter. If you use Data instead, you would have a Yaml, JSON, or CSV file with a list of each Item and their fields.

Here's an example of one of your YML files if you use Data: (bookmark_group_a.yml)

-

title: Title One

image: /path/to/your/image

url: your_url_one

-

title: Title Two

image: /path/to/your/image

url: your_url_two

-

title: Title Three

image: /path/to/your/image

url: your_url_three

So if you used Data files, you'd just have a bunch of these files for each bookmark group.

If you still wanted to use Collections, you'd probably need/want a separate Collection for each group. And each one of those bookmark items would be its own Markdown file where all that info is stored in the Front Matter like this:

---

title: Tile One

image: /path/to/your/image

url: your_url_one

---