why the ultasound image is noisy by speckle noise?
There are plenty of sources of noise in an imaging system. In the domain of sonar systems (which I work in) there is a big problem of reverberation, which is a fancy way of saying "sound from something that we aren't interested in. When using a sonar at sea, you get reverb from the sea surface, sea bed and the water volume. Taking volume reverb as a simple example, this is the water itself (or particles in it) returning sound to the receiver. The best way to relate to this is like driving your car on a foggy night. The sea bed and surface can absorb or reflect sound energy, depending on their composition (e.g. sea state). It's worth noting that this will bugger up your sonar image.

Another overlooked source of noise is in the signal processing itself. Your sensor is not going to be perfect and will quite happily distort both the amplitude and phase of the phenomena its trying to measure. In acoustics, you'll find microphones and hydrophones will be "tuned" for a certain bandwidth, and are quite useless outside of this frequency range. Typically your analogue to digit convertor will have some form of quantisation error with the values its spits out. It simply doesn't have the infinite precision required to represent measurements of the real world. Before we move off the subject, we should be aware that any electronics experiences interference from its  environment (e.g. induction, radiation, poor design). Add all this together and you have an estimate of what's called your "noise floor" and as a designer, you want to both minimise this by design and use other techniques to boost the signal.

Once in the digital world, your sample values are pretty much safe from additional noise. But wait there's more. In acoustics you'll want to electrically steer your array and spatially filter the incoming signal (look in different directions). This is also known as beam forming. The basic principle of beam forming is that you apply delays to the signals you receive and then sum them all together. Using the awesome power of constructive and destructive  interference, incoherent signals cancel out, and coherent signals emphasised. This isn't perfect either because the degree of spatial attenuation varies depending on a number of factors (e.g. array length and element spacing). It's worth pointing out that this aspect of processing features quite a bit of arithmetic on data types that have limited precision. Usually it's not noticeable but its there, trust me.