Adding custom image sizes and post types to a plugin or to a theme?

Question

I'm just getting started but want to play around in the right area practicing for a new personal site/theme.

Part A: I want to add featured images (and a few new image sizes) to a blog portion of a theme. It feels like all this should be done in the child theme itself, right?

Part B: I also want to add a portfolio page template with associated custom post type. (in a custom, plugin, correct?) Would it be ok to still include the page template in the theme?

Just looking for the best locations to add this, not the code itself.

Answer 1

New image sizes need new rules in the stylesheet, so the best place for them is usually in the child theme. On the other hand: attachments are content, so the answer to your question depends on the specific use case. If you need these images in back-end only, or in a part of the front-end that is not affected by the default stylesheet (a slider, a light box), they can be placed in a plugin.

Custom post type registration belongs to plugins. See Where to put my code: plugin or functions.php?. The template for this is usually a theme job.