For frequent posting this would be fine. For your main site images - you should focus on optimizing those separately and self host them.
Create a guide to using the Flickr app for your client so the client can generate and post these images when they want to with real content - their actual thoughts at the moment.
The goal of the guide would be not using the Flickr App, but using it to achieve the client's goal which you are facilitating. When your client takes pics, then they will be posted with useful parameters, metas, tags, and naming conventions. If you need to adjust them later you can always do that.
You can use a Flickr image or gallery to build content around on the site and link social posts to that page. This is what will help get your images ranked. You could self host a main category image on a category page about a spot and then build your content use Flickr images so the most relevant image will be your self hosted image if you optimize properly.
More benefits include ensuring fast loading images and good participation from your client which is what will matter in the end, with the pics being the same either way.
