I'm sure it is open to a bit of interpretation, but to my eye the grain pictured upper is much tighter and closer to parallel, evoking a sense of "vertical" straight from bottom to top of the bowl, while the lower picture is more loose and wavy.
Here's a nice little write-up on different grain types and classification: http://www.talkingtobacco.com/2013/03/a-grain-of-truth/
@whitewolf the blogger who wrote the article posted above would agree with you. Straight grain is tight and vertical grain, while flame grain flairs out from heel.
-- Pat
The word straight grain is overused and usually is an attempt to get a better price so you will encounter many examples of the misuse of the word. Unless the grain lines are parallel to each other through the entire defined area, it is not truly straight grain.
Coincidentally, I read the article Pruss referred this morning. If I were classifying, and if there was doubt, I would categorize as flame grain. I know the general preference is for straight grain but I've seen some awesome pipes with flame grain.