You can attach an image when you create an expense, or you can save an expense and attach the image later.
For more information on creating and updating expenses, see Mobile Expenses: Creating an Expense and Mobile Expenses: Editing and Deleting an Expense.
Images attached to mobile expenses are saved as Salesforce files. To use Salesforce files elsewhere in your org, enable the following Salesforce Files Setting:
For more information, see "Salesforce Files" in the Salesforce Help.