Settings like the image format and compression come from the defaultArnoldDriver node, so to set them through scripting, you need to set defaultArnoldDriver attributes.

The image format corresponds to the ai_translator attribute.

# Python
import maya.cmds as cmds
cmds.setAttr( 'defaultArnoldDriver.ai_translator', 'exr', type='string' )

# PyMel
import pymel.core as pm
d = pm.PyNode( 'defaultArnoldDriver' )
d.ai_translator.set( 'exr )

// Mel
setAttr "defaultArnoldDriver.ai_translator" -type "string" "jpeg";

The rest of the attributes are named a little… better 😉 Here’s a Python snippet to list the driver attributes and their values:

from maya.cmds import *
sn = cmds.attributeInfo( inherited=False, short=True, type="aiAOVDriver" )
for s in sn:
    print "defaultArnoldDriver.%s = %s" %( s, cmds.getAttr( "defaultArnoldDriver.%s" % s ) )