Photonic had support for the 500px API built in. In fact 500px was the third source supported by Photonic back in 2011 (after Flickr and the erstwhile Picasa). However, in June 2018 500px discontinued its API (https://support.500px.com/hc/en-us/articles/360002435653-API-?mobile_site=true).
I had contacted them at that point, and they told me that the usage of the API would require me to pay an insane amount (can’t remember if it was $5000 or $50000). Considering that I don’t make money off the plugin it seemed pointless to spend such big money. So Photonic stopped working for 500px about 2 years back.
