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

Question

I want to create a simple wordpress blog site. At the moment I will use a template, but I would like to develop my own afterwards. And I have one concern related to images.

Current template has nice big images and no sidebar, but in my future template I will definitely have a sidebar, which means that I will have to change image dimensions.

My main question - is there a good way to change image dimensions without using css? I wouldn't like to re-create all posts after I change template, because there will be a lot of them.

For example, images at the moment are 900px wide (height is not as important), but in the future template I plan that I will need to change image width to 600px. I would prefer not to do it with css's "max-width".

Perhaps there is a tool that can preserve original image and then re-create them with new dimensions afterwards?

Answer 1

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.