What technique can I use to remove useless information from an image?
Although you did not spent even a minute in researching the question I will post an answer to it. There are multiple ways; I will try to demonstrate it using wavelets in Mathematica.
So, first of all we need an image.

img = ExampleData[{"TestImage", "Mandrill"}]



  


Then we apply the DiscreteWaveletTransform using the HaarWavelet

dwd = DiscreteWaveletTransform[img]


We plot the transformation

WaveletImagePlot[dwd]



  


We threshold it, i.e. removing the noise according to some criteria

wtd = WaveletThreshold[dwd]


Again plot the transformation and the filtering

WaveletImagePlot[wtd]



  


We now inverse the transformation

iwd = InverseWaveletTransform[wtd]



  


And visualize the differences between the two images

ImageDifference[iwd, img]