When I take a picture, overexposed horizontal white lines go all across the LCD display

Nope, this is a hardware problem and the camera needs to be worked on. Notice how your display works fine but the finished product exhibits the anomaly. The recording end is messed up.

It's common on the G12. This is the place fixing these: www.CamerasAndParts.com They have a specific service package for this $89, parts and labor come included, plus they do the internal fix so it doesn't keep occurring on the camera. They are the best place for help with the Canon Powershots. Best of luck with it.

If your LCD TV or monitor has stopped working, or is displaying one of the following
symptoms, then it's a candidate for some new capacitors
- Flickering screen
- Screen image disappears after several seconds
- Dim screen
- Slow start
- Power LED on, but no picture
- Unusual colors and/or lines
The primary cause of LCD TV and monitor failure is caused by faulty capacitors. You can
examine the capacitors in your LCD TV or monitor and actually see if they are bad.
If they appear bulged on top, then they need to be replaced.
New capacitors will solve a host of problems in LCD monitors and TV's and will extend the life
of your monitor or TV by several years.

Take it to a repair shop.

Yes, this is caused by a defective LCD display, There is nothing that you can do to make them go away. It is caused by a defective transistor in the LCD display logic that is either stuck on or off. The transistor controls a single horizontal line across the screen. The only way to fix this problem is to replace the display.

http://www.sharc.net/gps_LCD_repair.htm

The camera has a censor problem, needs repair.

it means there is some dead pixel in the screen. you can change one lcd screen and buy then fix it by yourself.
here is the links for the lcd screen,maybe it can help you!
http://skiliwah.en.alibaba.com/product/475587936-210827720/LCD_Display_Screen_for_Canon_Powershot_A470_Digital_Camera.html

If I understand you correctly, you're saying that the LCD display shows a normal picture when you are composing your shot, but that when you actually snap the shutter to record the shot, the resulting image has nasty lines across it?

That would indicate that the lens is fine, but that the image sensor itself or associated electronics have gone bad. If this is out of warranty, then you're likely out of luck.

Black screen after clicking a picture means aperture motor not working or lens flexible circuit damaged, it has to be checked by expert technician & is replaceable part.

It is the shutter (or at least it was for me) I found a simple test here: http://www.mydc.com.tw/repair/knowledge_en/casio-exilim-ex-s600-overexposure-t17.html
I repaired it myself, but it took me a whole afternoon and lots of patience.
The hard part is taking the camera apart, once I got to the shutter I just activated it manually so to dislodge any pieces that might have gotten jammed, then placed it back together again and it worked! :).
It's really hard to take apart, it's like a puzzle sometimes, to find which screws are keeping it together. I also had a bit of hard time when it came to put the zoom back together again.
But I guess it beats throwing the camera away.
I'm still puzzled on how the CCD works so that an overexposure would make horizontal overexposed lines, but everything points to that. Fixed shutter, lines be gone.

I also having this problem white horizontal lines ...any solution?
If repair how much around?Please help!