I would like to send images as attachment by default in Inbox by Gmail. Is it possible to define this as a default setting?
No, not currently.
I recommend sending feedback to request this feature be implemented. Here is video on how to leave feedback for Inbox by Gmail: https://youtu.be/9J_DIGvaW3k?t=49s.
If more people take time to submit feedback, it is more likely that the feature will be implemented. To make it really easy, feel free to copy and paste the following:
Please provide an option to default to "Send as Attachment" when adding adding photos to emails. I would like to be able to default to "Send as Attachment" on desktop browsers as well as mobile apps.
I did not include a screenshot when I left feedback.