The Use Input control, which appears for multiple layer types of the Texture Synthesis dialog, controls whether the layer will operate on an image selected within the dialog itself, or whether the node's input will be used.
Within the Nodes tab, insert a node into the Synthesis node's input slot.
Next, within the Texture Synthesis dialog, activate the Use Input option.
If the image contains an arrangement of individual images, select each individual image with its own patch.