Is it possible to preserve original images in order to change image size later?

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Yes, this is trivial in WordPress.

Whenever you upload image WP keeps original file and generates additional sizes from it. Several of the sizes are built–in (see Settings > Media > Image sizes) and any additional ones can be added with add_image_size().

So when you develop your own theme as part of the process you:

  1. Determine image size you need.
  2. Register that image size and use it in the theme.
  3. WP will generate the size from existing originals for you.