Re: Image filter
The image filter usually is band pass filter and must be designed with the spec obtained from RF System Design process. In general it is just ordinary filter, but sometimes it may be something more exotic like multiple notches or resonance filters or filters with pads in order to optimize mixer termination (in this case it is necessary to keep in mind that thermal noise at the image frequency will be restored after filter). One more point: the image filter usually installed after the LNA and serve not only for image rejection but also for attenuation of products generated into LNA and improving the total system linearity.
Group delay is a kind of different parameter because it is not natural, but artificial. It is the measure of how dispersive the filter is. Actually it is important mostly for digital communications because it increase the ISI. More important not the absolute value of S21GD, but rather its variations. Historically it goes from old wired multichannel communication systems when channels were combined into groups. The first 12 channels were called the primary group. Then 5 such primary groups created the secondary 60 channels group, then 5 secondary groups combined to the tertiary 300 channels group and so on. When there are a lot of channels and groups the occupied spectrum become wide and signals from different groups arrive to the end station with different delay. From this phenomenon term group delay was derived. Mathematically it is slope of the phase response and related to the speed of energy transfer through the system. For today's communication engineering the group delay parameter is one the most important and must be carefully considered at the time of design process.