Introduction to Campaign Repositories
What is a repository?
A campaign repository is a set of files (mostly images) that are used in a campaign. By hosting a repository someplace other than the GM's computer you can help speed up the image download time of your players, making the game run smoother in general.
You must have some externally hosted web space to properly use a repository. Many ISPs give you some space you can use for this type of purpose. If your ISP does not provide this you can usually find cheap (and even free, in some cases) web hosting if you search around.
Unless you are running MapTool from a really beefy server-class machine on a T1 connection (or you picked a really bad web server) you're almost guaranteed that clients will be able to download files from any hosted web storage faster than they will from your machine. This is one of the main benefits of a repository.
Creating a repository
To create a repository file, open your campaign in MapTool and go to File -> Export -> Campaign Repository File. This will create a zip file that contains the repository information.
Unzip the file. (This is very important! MapTool cannot use the repository if it remains zipped!) Inside should be a folder called "assets" and a file called index.gz. Upload both the assets folder and the index.gz file to your web storage. Make sure they are in the same directory. (Do not put the index.gz file inside the assets folder. It should be at the same level as the assets folder.)
Make a note of the URL for to your index.gz file. You can test the URL by typing it or copying and pasting it into your browser's address bar. If you have the address right you should see a page full of letters and numbers that won't mean much to you.
In MapTool, go to Edit -> Campaign Properties then go to the Repositories tab. Delete any links that are already there, then add the URL of your index.gz file.
Save your campaign. Now when clients connect to your server they will get the campaign files from the repository online instead of your PC.
If you make changes to your campaign file you will need to re-export the campaign repository file and re-upload it to your web space. You will not need to re-add the URL to the Repositories tab of the Campaign Properties unless you changed the location where your files are being kept.