For override/extend the dialog when your component is inheriting from core Image V2
Copy the cq:dialog node from core v2 image to your image component
If you want the Asset and Metadata tabs and its content to be available along with new tabs,
Delete the asset and metadata node under /cq:dialog/content/items/tabs/items
add new tab items under this path.
If you don't want the Asset and Metadata tabs and its content to be part of your dialog,
let the node name be asset and metadata, you can choose to change the title of tab item (asset node for example) and its contents.
node named asset and metadata should be available which indicates we are overriding the respective nodes, otherwise the same will be available in your dialog as we are inheriting the component from core v2 Image.
Screenshot for clarity:
Only asset node is overridden, metadata tab is coming from Core Image V2 dialog
Make sure your tabs are under cq:dialog/content/items/tabs/items path. If you don't want the parent component tabs use sling resource merger property sling:hideChildren String asset,metadata to items node under tabs.