You have to inject a msg.filename with the way to the picture in.
Like this 2 examples:
[{"id":"3a1bff0b.55d3c","type":"ui_template","z":"a8b1b208.7036f","group":"c39cfd51.14f1c","name":"","order":1,"width":"6","height":"6","format":"\n \n <img width=\"100%\" height=\"100%\" alt=\"Image not found\" src=\"data:image/png;base64,{{msg.payload}}\" />\n","storeOutMessages":true,"fwdInMessages":true,"resendOnRefresh":false,"templateScope":"local","x":920,"y":1510,"wires":[[]]},{"id":"db2e5ea3.ed84","type":"base64","z":"a8b1b208.7036f","name":"","action":"str","property":"payload","x":770,"y":1510,"wires":[["3a1bff0b.55d3c"]]},{"id":"bc137f20.f932b","type":"function","z":"a8b1b208.7036f","name":"msg.filename","func":"msg.filename = \"/home/pi/Pictures/off.png\";\nreturn msg;\n \n\n","outputs":1,"noerr":0,"x":340,"y":1510,"wires":[["4757d887.183fc8"]]},{"id":"ee87f65b.9f4398","type":"inject","z":"a8b1b208.7036f","name":"","topic":"","payload":"","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":1510,"wires":[["bc137f20.f932b"]]},{"id":"4757d887.183fc8","type":"file in","z":"a8b1b208.7036f","name":"","filename":"","format":"","chunk":false,"sendError":false,"encoding":"none","x":570,"y":1510,"wires":[["db2e5ea3.ed84"]]},{"id":"2672a340.075b0c","type":"change","z":"a8b1b208.7036f","name":"pay to msg.filename","rules":[{"t":"move","p":"payload","pt":"msg","to":"filename","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":370,"y":1570,"wires":[["4757d887.183fc8"]]},{"id":"f4caf68a.661d88","type":"inject","z":"a8b1b208.7036f","name":"/home/pi/Pictures/off.png","topic":"","payload":"/home/pi/Pictures/off.png","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":1570,"wires":[["2672a340.075b0c"]]},{"id":"c39cfd51.14f1c","type":"ui_group","z":"","name":"camFoscam","tab":"250de740.57ee78","order":13,"disp":true,"width":"6","collapse":true},{"id":"250de740.57ee78","type":"ui_tab","z":"","name":"Dashboard","icon":"dashboard","order":1,"disabled":false,"hidden":false}]