According to your description, you want to have an image "button" only for the last record in your report, and use this "button" to call a subreport. Right?
In Reporting Services, we can add a subreport into a report and set the subreport toggled by a report item in visibility setting. However, this report item can't be a dynamic textbox in a detail row, it should be a static one. Since you only want the subreport
be called by the last record, we suggest you add a row out side of group, then set the subreport toggled by the textbox in the added row. We have tested this case in our local environment. It looks like below: