How to apply a transformation on multiple images?

Transformations can be added on the fly to any image just by updating its URL.
For example, let's say you've created a named transformation with the required thumbnail parameter and named it "my_thumbnail". All is needed is to set the transformation parameter to my_thumbnail (t_my_thumbnail in URLs) when generating the image's URL. For example:

http://res.cloudinary.com/<your_cloud_name>/image/upload/t_my_thumbnail/image1.jpg
http://res.cloudinary.com/<your_cloud_name>/image/upload/t_my_thumbnail/image2.jpg
...

Also, this can be done with the API, for example in Rails:

cl_image_tag("image1.jpg", :transformation => 'my_thumbnail')
cl_image_tag("image2.jpg", :transformation => 'my_thumbnail')
...

Sometimes, if the transformation is more complex it may take a few seconds to process before your asset can be delivered, to prepare your asset ahead of time, you can use an eager transformation. If you'd like to eagerly generate the transformation to all of your images, you can go list your resources, and use the explicit API and the transformations will be generated and stored in your account, ready for quicker delivery.