The EPS format version of the Cliptures World Flags is the best choice to use with Adobe Photoshop. The EPS flags are "vector" format graphics. Photoshop can use these to create flag images at whatever size and resolution you need.
For a more complete information about what is happening with the EPS vector flags, when you open them in Photoshop, see the note at the bottom of the page.
Here's how you do it:

From the Photoshop File menu, select the Open command and navigate to the drive and folder that has the EPS flag file you want to open. Select the file and click the Open button. Note: Photoshop is going to generate an image of the flag using the EPS flag file. It's not going to actually open the orginal file the way a software program traditionally opens a file.

Before Photoshop actually opens the flag file on your screen, it will display a Rasterize Generic EPS Format window. This is where you will specify the various setting you want the image to be.

Both of these boxes should be checked. Anti-aliased will insure that all transitions from one pixel color to the next in the Photoshop generated flag image are smooth. Constrain Proportions will keep the Width or Height in proportion to the other when you set the Width or Height you want the generated image to be.

Specify the Width or Height you want the generated flag iamge to be. You don't have to set both because the Constrain Proportions box from the previous step is checked. If you're not sure of what the exact finished size of the flag needs to be, make it bigger than you think you will need. (Reducing the size of the flag later will not hurt the quality of the image but making it larger might.)

Specify the Resolution you want the flag image to be. A resolution of 300 is common for desktop printers. It's up to you though. Photoshop will create a version of the flag and whatever resolution and size you specify (and that your computer's memory will handle).

The EPS flags are all specified as CMYK colors in their original files. Unfortunately CMYK colors will appear dull in Photoshop. In most instances you will want to specify RGB Color as the color model to use in the Photoshop file. Then, when you open the flag file, Photoshop will convert all of the colors from CMYK to RGB.
With all of the above setting make, click the OK button and Photoshop will create a "rasterized" (bit-mapped) version of the flag from the EPS vector flag file. The Photoshop image will be at the size, resolution and in the color model you specified.
Photoshop is generally a program for working with bitmap images but it does have some vector type features and, more importantly, it understands and can work with EPS vector images.
What happens when you open an EPS vector flag in Photoshop is that Photoshop first recognizes that the flag file is a vector type file and has the ability to create a bitmap version of that file at whatever size and resolution you desire (and that your computer system can handle).
You specify the size, resolution and color model that you want the flag to be. Photoshop then reads the EPS vector flag file and generates an opened copy of that flag with your specifications.
This process does not change the original EPS flag file. As long as you don't overwrite the original file, you can open the flag again and again and generate other flag images at the same or different sizes.