It's not "+" and "=" as a "rule", it's the "+=" operator, which does a similaar thing to what it does for an integer:
int x = 666;
x += 334;
Console.WriteLine(x);
Here it adds 334 to the existing value of 666 and prints the result: 1000
With an event handler, it does a similar thing: adds a handler to the event handler chain so that it gets called when the event is fired.
And no, a timer isn't "accurate" - it will happen "at some point" after the timeout period has elapsed, but your handler method isn't guaranteed to be called immediately the Tick happens. Windows isn't a real-time operating system which is the only time that timed events can be guaranteed to occur at a particular frequency.
I'd convert the images to a stream or video, and play that instead.