You can modify the system emails sent to bidders and donors by clicking on Communication -> Manage System Emails
To edit a specific template click the Edit link which will open it in the Email Editor
You can also preview each template using sample data.
The options link will let you customize the subject for each email.
You can use the normal Email Editor features to add text and images.
Many emails also have fixed blocks in them that provide the specifics of the particular email and these are not directly editable.
For example the bidder statement has blocks that show the items won, purchases, etc.
These are shown in the editor as a preview but can't be edited in detail.
Many of these blocks have a custom visibility set which is indicated by the eye icon
In the settings for this element you can see the condition that controls the visibility
These are used to ensure only the correct blocks are shown to the bidder.
Generally these conditions should not be modified.