Sometimes pictures are dark even in auto mode, err message is always coming, dark image in the view finder.

This is most often a problem with the imaging sensor and no reset is going to repair the part. Also, if replacing it, it needs to be properly calibrated after installed. If you need help with this, this is the place we recommend for the Canons: www.CamerasAndParts.com They are the best for these models, they only work on Canons and they are excellent for repairs. They have a service package for about $90 which takes care of an image problem as you have or you can try emailing them first. As far as service and the quality of work, I wouldn't go anywhere else but them for a Powershot. They are the best for these cameras. Good luck

Push your shutter 1/2 way down. See whats the readings in view finder.

Load up the ATG Canon EOS Utility on your camera & check by shooting tethered. Then view it on a big screen,

A stuck shutter is another common failure mode for digital cameras. The symptoms of a stuck or "sticky" shutter are very similar to CCD image sensor failure. The camera may take black pictures (for shutter stuck closed), or the pictures may be very bright and overexposed, sometimes with lines, especially when taken outdoors (for shutter stuck open). To confirm a stuck shutter, put the camera in any mode other than "Auto", and turn the flash OFF (you don't want to blind yourself for the next step). Next look down the lens and take a picture. You should see a tiny flicker in the center of the lens as the shutter opens and closes. If no movement is seen, then you likely have a stuck shutter. If so, please see this link for further info and a simple fix that may help.

make sure flash is on your flash on cam is sticking.

A stuck shutter is another common failure mode for digital cameras. The symptoms of a stuck or "sticky" shutter are very similar to CCD image sensor failure. The camera may take black pictures (for shutter stuck closed), or the pictures may be very bright and overexposed, sometimes with lines, especially when taken outdoors (for shutter stuck open). To confirm a stuck shutter, put the camera in any mode other than "Auto", and turn the flash OFF (you don't want to blind yourself for the next step). Next look down the lens and take a picture. You should see a tiny flicker in the center of the lens as the shutter opens and closes. If no movement is seen, then you likely have a stuck shutter. If so, please see this link for further info and a simple fix that may help.

did you try removing the memory card and making sure its not locked?

Here's a link to the N75 / F75 user guide. The troubleshooting section / lens compatibility area might point to the problem. If the viewfinder is dark, the mirror is likely stuck in the UP position. This is usually due to a lens compatibilty issue, or the fact that the lens is not set to its smallest aperture (ie 22). The camera needs that to control the full range of aperture openings

Could be many different issues.
What camera is it?
When you put a lens on it, is the view finder quite dark?

Try playing back pictures you know were captured properly ( either with another properly working camera or with your camera before the problem with quality has appeared ). If the image is ok in playback but 'noisy' in capture mode then the CCD sensor is faulty /on its way out. Ask Canon if they repair CCD faults on this model free of charge.