[anezvigin]

(edit: The above commenter deleted their post as I wrote my reply. They made the point that the traditional architecture of offloading email and image processing is suitable for background queues/workers and don't necessarily need to be services. This is very true; my reply points out when you might actually want to do it.)

They don't have to be, but they can be.

For image processing, you might to process (and cache) images on the fly. This is very, very useful for companies that manage a lot user uploaded media. Processing images in the background forces you to decide ahead of time what dimensions, formats, filtering, etc you want. Adding another image variant means running a backfill on all existing images. There are a few open source libraries that exist (I contributed to https://github.com/oysterbooks/halfshell, but there are others) and SASS solutions (https://www.imgix.com/).

For sending emails, I've built an API that does more than simply route emails to a 3rd party. The marketing team was given a simple interface to edit transactional email templates, A/B test them, track engagement, manage unsubscribe lists, and embed dynamic content. This service also sent SMS and push notifications.

It really depends.