[Patch]Fix old image flash when switching new one
[Patch]Fix old image flash when switching new one

Fix #286

stvreumi/electronic-blackboard,SWLBot/electronic-blackboard,Billy4195/electronic-blackboard,stvreumi/electronic-blackboard,Billy4195/electronic-blackboard,stvreumi/electronic-blackboard,Billy4195/electronic-blackboard,SWLBot/electronic-blackboard,SWLBot/electronic-blackboard,SWLBot/electronic-blackboard,stvreumi/electronic-blackboard,Billy4195/electronic-blackboard