Bicycle Mechanics - What size nipples?

I'm pretty new to biking and don't know much about repairs (yet). I tried Googling this one, but there seems to be no easy way to tell what size nipples (my rims) have.

Apparently I like going off curbs, as today was the second wheel true I had in less than 40 miles, so I'm thinking of getting a spoke wrench so I can do it myself. These are the rims I have:

http://img51.imageshack.us/img51/8966/74543500.jpg (http://img34.imageshack.us/img34/109/74687202.jpg)

Is it even worth buying a spoke wrench or can I just use needle-nose pliers?

Absolutely do not use anything other than a good quality spoke wrench to adjust the spokes. You will damage them and make it nearly impossible to adjust subsequently. Spoke wrenches come with several sizes to fit the nipple. Be careful in picking one out as the cheepies are often too loose and just round them off. If you're going to do that you might as well bugger them up with the needle nose pliers. In other words, get a good quality wrench that fits without play on the nipple.

Park sells three different sizes of their "loop type" spoke wrenches. The "Black" (SW0) is the most common size as all Wheelsmith, DT and other good quality spokes require it. The "Red" (SW2) wrench is used for older Japanese spoke nipples and the "Green" (SW1) is used for some unusual European spokes.

Take your bike to a bike shop and get the wrench that fits snugly on your particular spoke nipples. They are quite inexpensive.

As an aside, if you insist on riding off of curbs, you might as well use needle nose pliers to adjust the nipples since the rim will be damaged beyond use very quickly anyway. :)

If that was just a joke and the wheel is going out of true without abuse, the entire wheel need to be retensioned since that is evidence the spokes are way to low in tension. Find a bike shop with an experience wheel builder and pay him to properly true and tension the wheel and that problem should be solved.

Get Jobst Brandt's book "The Bicycle Wheel" and learn how to build and repair your wheels.
You will need either the black or red Park spoke wrench.

+1 on the need for a spoke wrench, one of many places that you can not use another tool.

The Spokey spoke wrenches (recommended by Sheldon Brown) are excellent and will not damage nipples of any type or size. The yellow or red (smaller) Spokey wrenches work on most common nipples.

^+1

Unfortunately the yellow spokey wrench (red Park Tool equivalent) is getting harder to find and it's the one I use the most, even for replacement spokes that you may have to buy at some LBS shop if you break one and works on the majority of bikes that I've run across. The red spokey (black Park Tools equivalent) gets used a lot less and is mainly for road bikes.

That being said, I would suggest buying both and you'll be covered for just about any spoke nipple size that's being used/sold today. The other spoke wrench sizes would be a waste of money, unless you own a bike that requires one.

The shop today told me it's "size 15" but I can't figure out how that equates to the more common sizes? Or are my wheels extremely uncommon?

I'm pretty new to biking and don't know much about repairs (yet). I tried Googling this one, but there seems to be no easy way to tell what size nipples (my rims) have.

Apparently I like going off curbs, as today was the second wheel true I had in less than 40 miles, so I'm thinking of getting a spoke wrench so I can do it myself. These are the rims I have:

http://img51.imageshack.us/img51/8966/74543500.jpg (http://img34.imageshack.us/img34/109/74687202.jpg)

Is it even worth buying a spoke wrench or can I just use needle-nose pliers?

Use only the proper spoke wrench to work on your wheels.
Most of my bikes use the red wrench, sized by color, but I don't know if they still do it that way because I bought my wrench 20 plus years ago and its still like new Park is the brand name and it Will pay for itself.

The moron at my LBS, where I had bought a new Trek 7.5 FX, told me the green Park spoke-wrench is what I wanted for Bontrager wheels supplied with the bike. My tools had been stolen - most of them - so I winced and bought the green one. When I returned to my shop, I quickly found that my black SW-0 fit much better. The green SW-1 was as loose as a goose. Glad I still had my black spoke-wrench.

Thus began a state-of-war between them and I. If it were not the Bontrager-brand crumby wheels, then it was that LBS pulling another idiot stunt. I have my full complement of tools I need now. And I wouldn't even buy an inner-tube from them.

Make sure your LBS knows what they're doing.