kokopelli, allow me to clarify :)
SexyBookmarks also serves all its files locally.
Read on --
SexyBookmarks supports over 80 services. An image sprite with all 80 icons is over 200kb in size. That is huge and will have slow load times (whatever time that it takes for your visitor to download a 200KB file).
Now, ground reality is that most publishers do not display all 80 services, but pick and choose the ones that fit their audience.
In order to reduce the size of the image sprite and as a result make everything quicker for your users, it had to be customized to the services that you specifically pick to display on your website and the order that you pick to display them vs. using a generic sprite with all 80+ services that is over 200kb+ in size and slow.
We had two options in order to achieve this -
a) either create a process that generates a custom sprite for you once on our server and save it on your server locally -- this happens once only when you save your settings in the admin panel. Each sprite is generated specifically and customized for the settings you choose.
(we picked this as the better option)
b) bundle every permutation and combination of 80+ buttons into individual sprites. It's been a while since I've done such math, but the number of combinations would be a few tens of thousands files to include in the install package, which is not at all practical and I'm sure will not be appreciated by you :)
One of the primary reasons we have to generate custom sprites (that are generated once on our server and then stored on your server locally) is that unlike other plugins our buttons are substantially bigger in size than other plugins. Bigger buttons = bigger file sizes. Bigger buttons also equal more sharing activity btw :)
We went through all the trouble of enabling custom sprites to make things a lot faster for you ultimately.