If you use the following FILE functions on the object containing the Document attribute then you could monitor a change on any one of these attributes. So if the file name or file size changes it would indicate a new file has been uploaded or deleted and your rule can run accordingly - (I am not sure if these functions will apply to a Picture Attribute)
Create 3 additional Attributes i.e. FileExtension, FileName,FileSize and then insert a rule on the object to update as follows:
Object.FileExtension=FILE_EXTENSION(Object.Document)
Object.FileName=FILE_NAME(Object.Document)
Object.FileSize=FILE_SIZE(Object.Document)
BTW, I see there is also a FILE_LAST_MODIFIED function which could work in the some way to be more precise