Copying from this post that also gives information about other stuff:
Important: The number of packages you select in the Shop defines how many packages you wish to have, rather than how many additional packages you wish to purchase. So, if you already have one package of userpics and then you choose ‘1 package’ in the Shop, only the time of your 1 package of userpics will be extended, rather than the quantity of userpics. If you’d like to have 2 or 3 packages in total, then you need to select either '2 packages' or '3 packages' plus the duration of time. Please note that the total amount of time purchased for all userpic packages will be redistributed so all packages expire at the same time.
Here's the breakdown on packaging and pricing:
And yes, I just might be absurdly happy about this :-)
ETA Sadly, as with so many thing that LJ does, the purchasing isn't as simple as it might seem - in fact many people have been caught out and LJ are looking at ways to make it simpler.
These two links the_cornettist shared: a useful thread and sums it all up where an LJ member of staff tries to explain may be of help to clear things up.
However, from what I can glean if you already have extra userpics and want more, it's no good just buying one package, as that is what, in effect, you already have. You need to buy two packages, but for a shorter length of time, i.e. to coincide with when your current set of extra pictures expires. That way you will get extra pics (at least I think that's the case).
But LJ are saying they are aware it isn't clear.
And also what isn't clear (to me) is:
1. What happens if Person A and Person B both decide to gift Person C with extra pics for a year. Will that person actually not get 140 extra pics, but only an extra 70 for two years.
2. What happens if a user buys one package for a year today then tomorrow realises 70 extra isn't enough and thus buys another package. They expect another 70, but it appears that isn't the case; they just get the extra 70 for another year. Thus, I'm guessing, they'd have to buy an extra two packages but for a shorter length of time. The idea being, apparently, that the packages expire on the same date.
So take care before you go ahead and buy extra pictures.