The best place to bleed the system is at the highest zone/radiator in the house, not at the boiler. First make sure the system pressure is at least 12-15psi, then go upstairs and look for the bleeder valves on the radiators/baseboards. (On baseboards it'll be under one of the end caps.) If you crack the bleeder and you can pull a quart or more out without any hissing or spitting, it's probably not an air problem.
You have an air vent right at the top of the boiler- the bronze thing on the tee where it branches off to the expansion tank, and even though it's not ideally placed, over time it should purge air from the system.
The expansion tank installation is not exactly a in installation of great beauty, but I've seen worse. By having it out 18" on a pipe it's less effective at dampening pump vibration, but it is also likely to create a leak point (or even a sudden catastrophic break) on that tee over time due to the large moment arm. That's a lot of mechanical stress to put on that tee!
The expansion tank SHOULD be before the input-end of the pump (the sucky side, not the blowy side) and in the shortest possible stub off the flow channel. Your installation seems to either have the boiler between the expansion tank and pump, and a 90 degree turn just before the pump, which maximizes the turbulence of the water as it enters the pump, OR (more likely) you have the impedance of the radiation loops between the expansion tank and the input to the pump, which is about as bad as it gets. The "percolating" you hear may in fact be cavitation on the pump due to improper expansion tank placement & implementation. This would be good to fix sooner than later, since cavitation will age the pump very rapidly.
The boiler looks almost shiny-new- was it just installed in the past year? If yes, the installer should be on the hook for re-configuring in such a way that it would at least get the gentleman's C pass in Hydronics 101.