Would that detail really be useful?
I think if the reason you want to keep a copy of the transaction receipt is for taxes or some other legal reason, you’re going to need the hard copy anyway. Since you’d already be keeping the paper receipt, what is the value of keeping a scanned image if it? CalendarBudget will already hold the correct amount.
I had thought originally to put some space to record some details for each transaction. My thought was it would be a textarea, where you could enter free text about any details you want. I ended up not implementing it, because it seemed overkill for the average user and I wanted to keep the edit dialog clean and not cluttered with extra fields that most people won’t use.