It is hard to find where you have added some referenced to items in Images and Documents module. If you use one single control, this will be easy, because you could get the control from the control collection and check the property that is used to set the link reference. Generally you could create a tool that will loop through all pages and search for a certain control type. Once the type is found you cold get the property that is responsible for setting the link/content/reference. Then by using RegEx you could match the link or dynamic url [provider]-Guid. Finally display all pages that has this reference and you could modify them manually.
This could be done when the content has just been deleted, if you subscribe for Executed event of ContentManager class and put your logic when the CommandName is delete content.
Right now this is not implemented in the beta
version of Sitefinity 4.0.
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