First let's start with this fact that you can have multiple files attached to one SharePoint record. because of this, you cannot bind the attachments to a single image control. Think about showing a table of attachments in one control, it's not possible. but certainly you can show the attachment(s) of a selected record, in a new gallery. Follow my example below:
1- Create one gallery for your SharePoint list, and create another gallery for your SharePointt attachments. align these two galleries side by side, or however you like to design your form.
2- Change the Item property of your second gallery to firstGalery.Selected.Attachments

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