Manually creating a lowpass filter from the frequency domain: what phase?
Typically if you're designing to an amplitude response, you set the phase at zero.  In processing time-domain signals, this is to keep the group delay of the filter constant.  In processing images, this will keep the filter effects centered, rather than displacing different spectral components of the image with respect to one another.
At least in time-domain processing there can be good reasons not to do this.  I can't think of any reason you would want a non-constant delay filter in image processing.
Keeping the phase at zero makes a constant-delay filter.  If you're doing your filtering after the fact, then you can just delay everything else by the same amount so that it appears that you're using a zero-delay noncausal filter.
However, if you're responding to the filter output in the real world (usually for some sort of closed-loop control, but possibly for fault detection), then that delay can be a performance-killer.  In addition, there's a feeling in the audio community that unwisely-designed constant-delay filters can cause aesthetic problems (Google on &quot;pre-ring&quot;).  You solve these problems (and cause others) by using a minimum-phase filter.  These are almost always done by using an IIR filter rather than an FIR.