the payload you have to inject has to match the value of the dropdown - not the label.
[{"id":"c0cbd894.a69f48","type":"inject","z":"c929e2e7887a5d65","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"},{"p":"options","v":"[{\"Camera-1\":\"cam1\"},{\"Camera-2\":\"cam2\"},{\"Camera-3\":\"cam3\"},{\"Camera-4\":\"cam4\"},{\"Show media 2\":\"med2\"},{\"Show media 5\":\"med5\"}]","vt":"json"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"cam2","payloadType":"str","x":130,"y":500,"wires":[["e05207bf.b24e1"]]},{"id":"e05207bf.b24e1","type":"ui_dropdown","z":"c929e2e7887a5d65","name":"Main_Display","label":"","tooltip":"","place":"Select camera source>>","group":"8712765e.3efe98","order":3,"width":0,"height":0,"passthru":true,"multiple":false,"options":[],"payload":"","topic":"main_display","topicType":"str","x":320,"y":500,"wires":[["9786b516.47a1b8"]]},{"id":"9786b516.47a1b8","type":"debug","z":"c929e2e7887a5d65","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":470,"y":500,"wires":[]},{"id":"8712765e.3efe98","type":"ui_group","name":"Main_Display_Area","tab":"f8095777.501678","order":3,"disp":false,"width":"12","collapse":false},{"id":"f8095777.501678","type":"ui_tab","name":"Home Surveillance System","icon":"dashboard","disabled":false,"hidden":false}]