Once you have uploaded your receipt photo and set the receipt's amount, DoubleAgent shows you any matching unexplained transactions.
First click It's for this one by the appropriate transaction. Then fill in the explanation fields as normal. When you press the Explain button, DoubleAgent will explain the transaction in FreeAgent, upload the receipt, and attach it to the explanation.
Here I have uploaded a train receipt for £32.10. DoubleAgent is showing the matching unexplained transaction.
I have clicked the It's-for-this-one button, filled in the Description, and set the Category.
As soon as I click the Explain button, the transaction will get explained in FreeAgent with the receipt photo attached.