Which vents are appropriate to put air filters on (pics)?

sorry, kind of amateur question, but i want to get cleaner air, and i don't know which vents can house air filters?

please see pics.

kitchen ceiling: http://tinypic.com/r/21a0as/9

sears pilot free (furnace?): http://tinypic.com/r/206g5mw/9

possible swamp cooler vent: http://tinypic.com/r/inai42/9

bedroom ceiling: http://tinypic.com/r/sn09ro/9

submitted by /u/harryloweishome
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