The simple answer is... the total amount of storage used (peak) during the month is the basis for cost. The amount of storage at any time is indicated by that graphic bar on the Images Home page. Under that bar is a "More Info" link that will show you peak use.
At 38.24 MB, you're a long way (at less than 4%) from 1000 MB = 1 GB limit before you need to pay more (unless your peak use at some point since the beginning of the month exceeded the limit). Unless you have over about 10,000 images stored at any given time for your operation, you're probably fine (never get there).
I would recommend you organize your images around use to help keep track, e.g. Store Inventory that never should be deleted in an appropriate Folder.
Hope that all makes sense.
Danno