Ok here's Eggy Lippmann's Guide to the Galaxy, I mean Texture Baking, or rather "OMG WTF MY AV WONT REZ".
Basically, odds are it's actually not their fault - Texture baking happens solely on your computer and as such it is not a server-intensive process

Your avatar's textures - skin, clothes, tattoos, makeup, whatever - go through a process called "baking". Here's how it works:
1) Your client has to download all the individual textures from the server
2) Your computer uploads the textures to the graphics card and creates a composite "baked" texture
3) Your computer has to UPLOAD the new texture back to the server
4) The server has to distribute that texture to everyone else
This means that a lot of shit can go down that is COMPLETELY out of Linden's hands:
1) Part of the internet path taken by the textures may be slow, or unreliable regardless of your individual connection speed. Data can go through 30 individual computers and network connections, and each of those can fail. It doesn't magically go from the sim to your screen.
2) If there's something wrong with your card, and it corrupts the texture, that corruption will be uploaded visible to everyone. So if you see a fucked up avatar, it's not your fault, or Linden Lab's... it's theirs.
3) Consumer-level ISPs have really crappy upload rates. It's not uncommon for your internet connection to upload 10 times slower than it downloads. It may take a LONG time for the freshly-baked texture to arrive at LL's servers.
4) See 1) - The internet is not flawless. Shit happens. It may be neither your fault nor Linden's... some crazy hacker may be spreading a virus or performing a "denial of service" attack, flooding an internet neighborhood that your data must traverse in order to reach LL

OMG WHAT TO DO!??
1) Don't run FILE SHARING apps! They eat your upload!
2) Ask your ISP how much UPSTREAM bandwidth they allot you, whether or not they throttle it, and whether or not you can get upgraded to a better plan...
3) Click on Help > About, and get the network adress for the sim you are in. It should be on the third line and look something like "sim738.agni.lindenlab.com".
Click on Start, Run, type in cmd and press enter. You are now on a SCARY BLACK SCREEN! Type in tracert sim738.agni.lindenlab.com and press enter. It should say something like "Tracing route to sim738.agni.lindenlab.com [69.25.105.170]
over a maximum of 30 hops:"
Wait a while, and then copy paste that into a notecard and hand it to your favorite techie... he will see what's wrong with your network connection
