The reason for blurriness is that when the image is increased in size there, it is not increased in perfect numbers, ..
If you set the main resolution to 100x100, it will look fine on 200x200 and 300x300 ..etc, but it will not look fine on 150x150 or 160x160.
If you want to fix the resolution from the start, then pick the smallest possible, which seems to be (half of) 320x240, this way, there is a higher chance of it scaling well when increasing its size.
You should consider making it resolution independent, your application should detect the resolution and handle display accordingly.