Since it's convex lens there in our eyes so image formed on our retina is inverted, so how come that we see upright images?
This is more of a psychology question. After you start seeing things, you notice that a certain side of your vision is the part that will see your finger if you touch your forehead, and the other side will see your finger if you touch your lips. We designate the first as "up" and the second as "down".
The brain just gets a bunch of signals. "up" and "down" are artificial tags we attach, where "up" is the side of our forehead and "down" is the side of our lips.
The brain turns it around. It can also adapt to other alterations of the visual field. It has nothing to do with physics.
I've heard several times the explanation that since the image is inverted on the retina, the brain has to turn it around somehow. This comes from intuitively and probably unconsciously picturing the head as something like a cabin where the brain lives, and sees on a display the image collected from the retina, image which it therefore has to turn around, in order to see it upright. But does the brain have an up or down sense?
What if we extract the brain from the skull, without disconnecting it from the rest of the body, and turn it upside down? Would it see the image upside down?
Let me put it differently. If the image would not be inverted (actually rotated), so if the mechanism was different than that of a camera obscura, nobody would have ask why we see upright. But if the image is inverted on the retina, we think that the brain should invert it back. But back with respect to what? There's no reference in the brain with respect to which the image on the retina is inverted. Why should the brain apply an algorithm to invert the image?