You can resize (or crop, which is another common use-case) the image at different phases:
- When the image is uploaded to Tridion ("baked" in the Content Manager)
- When the image is published to Content Delivery ("baked" on publish)
- When the image is delivered to the visitor ("fried" on request)
Each of these approaches has their own advantages and disadvantages. For example: when you resize the images after uploading (phase 1), you can inspect the resized images in the CMS and if necessary improve it manually. But you have derived content in your CMS (which is a form of denormalization) and the editors will have to pick the correct image size for their situation (error prone).
Consider limiting the allowed Schemas to a specific Multimedia Schema to reduce errors if taking this approach. The uploaded image could be one Schema (e.g. named "High Resolution Upload" or similar), possibly set as the default for
Images. The resized image could be a different Schema (e.g.
Default Multimedia), which the News Article allows in its image field. CMS editors would only be able to pick the right type of image.
Nick offers solutions that work in phases 2 and 3.
If you want the image to be resized when it is uploaded to Tridion (phase 1), you should probably look into writing your own event handler that triggers when the image uploading is completed and then generates the derived images.