You can convert it into a hash by using
JSON.parse(json)
and reconstruct the json without the keys
json_hash = JSON.parse(json)
inner_array = []
json_hash["images"].each do |elem|
inner_array << elem.collect{|key, value| value unless ["file_name"].include?(key)}
end
json_hash["images"] = inner_array
json_hash.to_json