checkimages.py: pyflake fixes

local variable 'usage' is assigned to but never used
local variable 'image_to_resub' is assigned to but never used
local variable 'pos' is assigned to but never used

Change-Id: I35b7ef6a8eefe3861817b2877d97e35b6ce0c5ea
Merge "checkimages.py: pyflake fixes"