If a camera lens is round,why are the pictures square?

We only see the part of the image that lands on the recording medium, whether it is film or a digital sensor.

If you want to try something interesting, put a "DX" or "Di" or other "digital only" lens on a film camera. This will demonstrate to you that the image created by light coming through a lens is indeed round. The diameter of "DX" or "Di" lenses is smaller than adequate to fill a full frame of 35 mm film. You will observe "vignetting," or a rounding of the corners of the image. This is because the actual image is round and with a "DX" lens, the borders of the image are too small to go beyond the edges of a normal frame of 35 mm film.

This works the other way around when you put a "film" lens on most digital cameras. The actual image created is so much larger than the sensor that there would never be any evidence of rounding. You are only capturing the center part of the actual image. This is actually a way to get better quality images. If you use a "film" lens on a digital camera (other than one with a full-sized sensor), you will be gathering light from the center or "sweet spot" of the lens. Most distortion is caused closer to the edge of a lens and this part of the image is way out of the field of detection on the digital sensor.

some cameras can make circle pictures

If you look closely into a camera's len you can see a square shaped figure in there. Does that answer your question?

The sensor (digital) or film on which the image is captured is a square or rectangle.

A few different reasons:

1. It lets more light into the lens
2. The Actual aperture is square (rectangular really)
3. All screens and monitors that you see pics in are not round
4. It's easier to calculate dimensions for squares pics

Your eyes are round(ish) but your see a rectangular world....

Lenses are round because of the way light behaves when transmitted through a medium.

Coincidentally, making lenses round in manufacturing, makes it much easier to assemble since round lenses are consistent no mater the orientation.

The reason the images are rectangular is because the film used is in a roll and using all the available film space makes sense when a photographer is trying to capture the best image. Roll film includes 35mm and 6cm. Sheet film; 4x5 and 8x10.

Because it focuses on film pulled through a rectangularplane or on a ccd that is rectangular

This is a funny question ...but lets consider it a serious one.
Yes the lens looks round and the final result a photo on the paper or digital file is a square.
That because:
1. For digital - the sensor is a square.
2. For film photography (35mm or else) - the film frame is a square.
3. When you print from film or digital the paper is a square. If the paper shape will be a star , well the photo will be as a star shape.
;-)

You could use Ellipse tool (Round images) in Adobe Photoshop.