Once you have uploaded your receipt photo and set the receipt's amount, set the Type dropdown to Out-of-office expense.
Then fill in the fields as usual and click Explain as an out-of-office expense.
Here I have uploaded a taxi receipt for £12.34. I have set the Type to Out-of-office expense, filled in the Description, and picked the Travel category.
As soon as I click the button, the expense will be created in FreeAgent with the receipt photo attached.