There are a few configuration options already in place for AI image captioning:
Forum admins can turn on AI image caption features through the setting:
ai_helper_enabled_features:
This allows for the 
Caption with AI button to appear over images.
You can restrict the “automatic” image captioning feature to specific groups with ai_auto_image_caption_allowed_groups setting:
Users in this group can optionally turn it on/off in their user preferences.
To save on tokens you could perhaps only allow a specific group of users to auto caption images (i.e. admins).
Still, not all images will be automatically captioned. There is a criteria for deciding whether an image will be captioned:
- image cannot be small (≤ 0.1 MP)
- image filename is less than 20 chars
- image filename is a single word
We don’t have the ability to customize that criteria at this time.