Magento can create different sizes of images, preset and even custom sizes. The images are cache by magent and can all be accesses progromatically.

stackoverflow.com/questions/14155385/how-to-get-full-product...
How to get full product image url in magento ,i need to migrate the data from magento to django so i need to get the product full image url to migrate the site

astrio.net/blog/images-resize-in-magento/
In this article I will show how to use re-size parameters of default Magento images re-size feature.

stackoverflow.com/questions/16476571/issue-with-resizing-the...
I am trying to resize image in product view page, but after the image has resized, white spaces come along in left and right both the sides (width), in resize function I gave the value 338,474, but It comes with white spaces. For that I applied functio ...

blog.chapagain.com.np/magento-resize-image/
You can resize image with fixed height and variable width. Or, you can resize with fixed width and variable height. Following code shows how you do it in Magento.

www.demacmedia.com/magento-commerce/varien_image-image-manip...
Varien_Image class s a great way to manipulate images in Magento. Here's a quick rundown of its various functions and how you can use them.
This is an indepth article and is easy to use.