Defined size category images - How?

I have created new images for each of my product and category pages.
How do I implement the facility for ZC to utilise my Category images?

You open up the CATEGORY to edit it...

ADMIN>>>CATALOG>>>CATEGORIES/PRODUCTS

Navigate to the desired level of categories, then click the green e button on the right, for the category in question.

It will be evident when you get to the edit screen where and how to apply the images.

Image dimensions are controlled in ADMIN>>>CONFIGURATION>>>IMAGES

Thanks fc for reply.
I have a 300 x 200 px image named ap_foo_MED.jpg in the 'medium' folder.
I have a 500 x 300 px image named ap_foo_LRG.jpg in the 'large' folder.
On my product page the 500 x 300 image is displayed, as desired.
When I have the apropriate subcategory selected in the category menu the main display area displays a row for each entry in that subcategory.
Each row displays a small image of 100 x 60 px dimensions.
It is this image that I am wanting to control the size of.
I suspect that I may have created an invalid situation through my ignorance of the correct method of achieving what I want.
I have several identical XAMPP and ZC installations for testing/learning purposes.
I am hoping that with the above information you might be able to ask me the right questions to track down the problem, or alternatively describe how I should have done it correctly.
The area that you have pointed me to contains the Product Image window and the Upload to directory dropdown menu.
I have used them to install my two defined images into the 'medium' and 'large' directories.
What dimension settings in ADMIN >>> CONFIGURATION >>> IMAGES should be selected/defined to achieve my objectives?
Changing the first two settings changed the size of the images displayed in the New Products Sidebox but not in the desired centre of page category entries.
How can we proceed?
Many thanks for help so far.

As mentioned above:-

ADMIN>>>CONFIGURATION>>>IMAGES

Look at the settings for:-

Subcategory Image Width
Subcategory Image Height

The settings here (as is the case with all image settings) are influenced by the RELATIVE ASPECT RATIO of the actual image used.

For example, if you have:-

Subcategory Image Width ...................... 100
Subcategory Image Height ...................... 50

(... indicating preference for LANDSCAPE images as width > height)

Then this will NOT allow an image to be displayed larger than 100px wide, OR 50px deep.

Now, if the image you use is, for example 200 X 90, then the RELATIVE ASPECT RATIO is 100 X 45

So, the image will be displayed at 100 X 45, because it HAS TO comply with the largest parameter in the settings.

If however the image you use is, for example 200 X 150, then the RELATIVE ASPECT RATIO is 100 X 75

In this case, the 75 is TOO LARGE for the setting's specified max height (50), so the image will be reduced accordingly to (approx) 75 wide and 50 deep.

The key is deciding the ASPECT of the images you want to use (either PORTRAIT or LANDSCAPE), then configure the display settings and base your settings on the RELATIVE ASPECT RATIO principle described above.