Flickr is one of the popular photo-sharing platforms. It's especially interesting because each user gets 1TB of storage to post their photos. That equates to more than 500,000 high-resolution photographs. So, if you've already got your photographs online at Flickr, there's really no need to upload them again at Posthaven. All you need to do is capture the link to the image(s) you want to display, then paste it on a line by itself in your post.
Find the image you want on Flickr, then click on the Share icon to display your options. Now click on the Link icon to display the shortened link, select and copy the link. At this point you can either paste that link on a line by itself in an email message, add a subject line and send it to your Posthaven blog or use the web editor to paste that link into a post - again on a line by itself. Publish your post and Posthaven will take care of the rest.
You can email multiple photos - each link residing on a line by itself - but Posthaven displays them as inline images rather than as a gallery.
One note, don't try to use Flickr's email option - also found as a sharing option. That email is sent from an automated service with an email address Posthaven doesn't recognize as an authorized account for your blog.