It is a great idea to compress your digital files when submitting them over the internet.
Compressing files is similar to wrapping your priceless china in bubble wrap before shipping it overseas.
Without the bubble wrap, your priceless china might end up shattered and worthless.
The same can hold true with digital files. As your files bounce around the internet, they can get stripped of vital code that identifies them as valid TIFF's or EPS's or whatever the original file types. Without compressing your files, you run the risk of sending us incomplete or "corrupted" digital files.
How to compress your files:
Simple Compression: (Windows XP, Vista)
If you are using Windows XP (or Vista), compressing files is as simple as right clicking.
To compress a folder or file, just right click, then click Send To, and select
"Compress (zipped_) Folder."
This will automatically create a zip folder in the same directory as the file (if the file is on the desktop, the zip file will be on the desktop).
Extracting is really easy as well. All you have to do is right click and select “Extract All.”
The folder will be extracted as a separate file in the same directory.

Simple Compression for OSX users (10.4 and above):
To compress a file, simply hit Ctrl + click (or right click if you have a 2 button mouse).
For users using 10.5, it will look like this:

For users using 10.4, it will look like this:

Extracting a .zip file in OSX is even easier. All you have to do is double click the .zip file and Archive Utility will automatically extract the folder as a separate file.
For all other versions:
You will need either
StuffIt Deluxe (Mac) or
WinZip (PC).
Using DropStuff (Mac)
Be sure that all your files are in one folder. Select that folder and drag and drop it onto the DropStuff icon. You may be prompted by a dialog box. Simply click "not yet". A ".sit" (or stuffed) file will be created. This is the file we need from you.

Using WinZip (Windows)
Be sure that all your files are in one folder. Select that folder, right-mouse-click, and pull down to "Add to (file name).zip". You may be prompted by a dialog box " simply click "agree". A ".zip" (or zipped) file will be created. This is the file we need from you.

Note that there are other ways to compress your files including ".sea" (self-extracting archive), ".tgz" (Unix file compression), ".dmg" (disk image), etc. At Modern Postcard we do not recommend other file compression methods.