Yep, it's way to hard to tell from a pic. Take a look at the pic below. What would your guess be? If someone posted this pic, saying they bought an 1/8th and asking if they were ripped off I'm sure 90% of people here would say "yes". It weighed out to 3.1 grams, just really dense dank. Ok, ok, an 1/8th should be 3.5 (it actually came from a 3 bud 1/4 ), but who would think it would even be close?

Best thing to do is to buy an inexpensive scale off e-bay. Not only will you know you're not getting ripped off but it will also end those "your 1/2 is bigger than mine" arguments with your friends when you split a bag.