There are lot of sites, which compare digital cameras by parameters, user experience and etc.
I looking for a website which has some kind of procedure to test image quality and compares digital cameras by the result. Are there any?
The website dpreview.com has detailed reviews that include sample images. You can preview sample images from various models to get an idea of the results each produce.
Also, check out flickr.com you can see images sorted by camera that took them as well.
(The links I provided are for viewing results for the Nikon D90)
DxO Mark provide objective data on the output of camera and cameras in combination with lenses.
The data comes from the testing they do on cameras and lenses for their RAW developing software "DxO Optics" so it should be fairly unbiased. I don't know enough about the technical aspects of digital photography to comment on the validity of their methodology.
It probably doesn't matter. Taking great looking photos has much more to do with the photographer than the camera. A good photographer can take great photos with any camera.
I suggest you decide what kind of camera to get based on the kind of shooting you do or intend to do, then visit snapsort.com to find a good model in that class. If you're going to take a lot of low light photos, get something with a fast lens; if you'll be shooting over a long distance, get something with a good zoom, and so on.
However, I agree with the suggestion to visit dpreview.com. They have extremely comprehensive reviews, which includes image quality and comparisons against other cameras.
First off, comparing images on the Internet is a misleading business since the resolution is usually too low to show up differences. Images from my old 2.1 Mp Canon P&S look not much inferior to my present 15 Mp DSLR when displayed in a normal web page.
However DPReview.com have, very fortunately for us, had a policy over many years of taking photos of the same resolution chart, under the same conditions and storing the original image for download. It is a marvelous resource. There you can find resolution images going back many years for all the major camera models.
Technical image quality has several dimensions and all need to be measured if you want a complete result. For many people though, resolution and chromatic aberration are the most important results. Luckily these things are easy to measure if we use the DPReview.com resolution image library.
Imagine we want to compare the following cameras (selected for no particular reason)
You can find the converted raw images under the Resolution heading of the camera reviews:
Horizontal edge, 15% from the center.
Edge blur in pixel (chromatic aberration in pixel).
For reference - 1.27 pixel is the best attainable result.
Nikon D7000........2.5 px (0.8 px)
Nikon D5100........2.6 px (0.6 px)
Nikon D3100........2.1 px (0.7 px)
Canon 600D.........2.4 px (0.6 px)
Panasonic DMC-G2...2.0 px (0.2 px)
I show edge blur because it is immediately understandable to most people. Reading MTF graphs is challenging. The important thing to note is the ringing in the graphs for Canon and Panasonic. This indicates that the images have been sharpened more than the images for Nikon, which distorts the results. Ideally the images should not have been sharpened, but we have to work with what is available.
The graphs are shown below:
It's impossible to objectively rate image quality, as image quality is quite subjective. You could look at example images, but they probably won't reflect the situations you would use your camera in.
Thankfully, most modern cameras have excellent sensors that outresolve all but the most demanding of photographers.
This tool lets you compare reference images from digital cameras side-by-side. These images are JPEGs straight from the camera, and were taken under carefully-controlled conditions, to provide valid comparisons of camera capabilities in actual shooting situations. You can also download the images (using your browser's "save image as" function) and output them on your own printer, to see how the cameras involved will perform in your application.
http://www.kenrockwell.com/ is a bible for choosing a lens or a camera !