QGIS Print Composer has tool "Save As Image" and I found appropriate function in QGIS source code exportCompositionAsImage. Is it possible to get access to existing Print Composer from Python console and perform this operation - export composition as image with composer settings.
After help from @ndawson, I'm using printPageAsRaster
function for printing different combinations of layers created with layerCombinations plugin:
from layerCombinations import classFactory
lc = classFactory(iface)
lc_manager = lc.manager
combinations = lc_manager.combinationsList
composition = iface.activeComposers()[0].composition()
composition_items = composition.items()
for cmb in combinations:
for item in composition_items:
if item.type() == 65641: #this is the type of ComposerMap
lc_manager.applyCombinationToMap(cmb, item)
image = composition.printPageAsRaster(0)
image.save('/home/dr/' + cmb +'.png','png')