Why It Is Showing "Object Reference Not Set To An Instance Of An Object" If I Browse Background Image From Propertybox In Vb.Net?

We can't answer this: you have done something to your form or controls which means that it's no longer fully valid to Visual Studio - so when it tries to access it, it fails.

Try shutting down all instances of VS and reopening your solution - sometimes VS can throw some very odd errors which this can resolve.
If that doesn't fix it, try looking at what you have done since it last worked and see if removing that fixes the problem. if it does, add it back slowly until you get it back again.

If that doesn't fix it, create a new solution and slowly add your existing code to it until you find out which part causes the problem. You can then look in more detail at that code and hopefully resolve it.

But we can't do any of that for you!