I have a forum where I'm only allowing Guests to view attachments on some boards.
To achieve this I disabled the 'View Attachment' permission for Guests in the default board permissions profile, created a custom board permission profile with the 'View Attachment' permission enabled for Guests, and assigned this custom board profile to the relevant boards.
1. Modify the Default Board Profile Permissions to prevent guests from viewing attachments
Administration » Permissions » General Permissions
Select the checkbox for "Guests", select "Clear Permission" for "View Attachments" and click on "Set permissions"
(If the "View Attachments" permission was enabled for Guests and has been cleared the number in the "Permissions" column for Guests will decrease by 1)
2. Create a new board permission profile
Administration » Permissions » Edit Profiles
Create a new profile with the name "Guests View Attachments", copy permissions from "Default" and click on "Create"
3. Edit the new profile and add the permission to allow guests to view attachments
Click on the newly created profile with the name "Guests View Attachments" to edit the permissions for the profile
Select the checkbox for "Guests", select "Add Permission" for "View Attachments" and click on "Set permissions"
(The number in the "Permissions" column for Guests will increase by 1)
Attachments GVA-3, GVA-4 and GVA-5
4. To change permission for an existing board
Administration Center » Boards » Modify Boards
Click on "Modify" for required board (eg, 'Sample Images') and change Permission Profile to "Guests View Attachments"
Attachments GVA-6 and GVA-7
5. To see which boards are using the Permission Profile "Guests View Attachments"
Administration Center » Permissions » Board Permissions
If ANY changes (eg, a permission is changed or a mod is installed that adds a new permission) are made to the "Default Board Profile Permissions" (accessed via Administration » Permissions » Edit Profiles) for ANY membergroup or a new membergroup is added AFTER you've created a custom board permission profile you MUST manually edit each custom board permission profile and update the topics, posts, general, polls and attachments permissions for each membergroup.
This MUST be done to ensure the board access permissions for all membergroups are consistent across al boards.
That said, for a custom board permission profile like this that only has one change it may be easier to create a new "Guests View Attachments" profile, assign it to the relevant board(s), and then delete the old custom board permission profile (Steps 2-4).