Doesn't matter if the pins are at the bottom of the keyway, bumping relies on the transference of impact energy... gravity has nothing to do with it.
And no, you don't need diferent tools to pick locks which have pins on the bottom. The feedback from the pins is a little different, but it makes no odds when it comes to how you actually pick them. I used to think that because the pins were at the bottom it meant that they'd overset more (and more easily), but that was because I didn't understand the binding defect properly. If you're not 100% on tolerance errors and what binding is, do some reading... you'll thank yourself for taking the time, trust me.
The only difference between locks with pins at the top and those with pins at the bottom is the feedback. Pins at the bottom will feel solid when set, pins at the top will feel exactly the same except you'll be able to push the key pins up slightly before you hit that stop. It's a pretty universal formula when it comes to pin tumblers... if a pin feels springy then it's not set, if it feels solid then it is.
Of course if they all feel solid and the plug isn't rotating, you have security pins and need to check each one until they're set correctly... that being said, false sets are very easily detected unless you're dealing with serrated pins which have shallow grooves.
It's also worth throwing in that if they're all springy and won't seem to set, then you've more than likely overset one of the pins, preventing the next pin from binding. It's a good idea to count the pins as you lift them, that way if you have a solid pin and 4 springy ones you know you've overset one. Another reason for pins not setting is that you're using too little tension, but as a beginner this is very unlikely since newbies generally use way too much. I know I did.
Hope this helps mate, I kinda went on a ramble there. Too early in the morning for me, I think.
Oh yeah, before I forget... you need to count the valleys on the key to see how many pins there are, it's hard to explain without a pic so if you're unsure what we mean by valleys you could stick a pick in there and push them all down, remove it slowly and count the clicks. Never fails!