You must use a Property Node within your subVI to modify the properties of an Image Display control in your main/top Level VI. Property Nodes are Block Diagram objects used to interact programmatically with Front Panel objects.
Property Nodes within a subVI can programmatically read or write to an object on the calling VI's Front Panel by accepting a Control reference input bound to that object as an input from the calling VI.
Figure 1: Main VI with an Image Control Reference to the Image Control
Within the subVI, a generic reference Control should be placed on the Front Panel. This can be converted to an Image Control reference using the To More Specific Class function.
Once the reference is converted, a Property Node on the subVI's Block Diagram will correctly identify properties associated with an Image Control. By setting the Value property, the image being shown on the calling VI's Image Display can be changed.
The figure below demonstrates how this can be achieved.
Figure 2: SubVI Acquiring and Writing Image to the Property Node.
A note for users of NI-Vision 7.0 and earlier: This technique does not work when using NI-Vision 7.0 or earlier versions. For these older legacy versions of NI Vision it is recommended to use a global variable to pass frames between the Main VI and any subVIs performing acquisition. This method requires the use of a while loop in the calling VI continuously polling the global variable for new data.