Ah yeah, that was dumb, I should've specified that you can do that, if you're looking at the post in the Lemmy web-UI.

The button would take you to the webpage of the original post, so before it got federated. Images often load when I do that, because my instance has an image proxy that's sometimes at fault for loading errors. If I'm interpreting your screenshot correctly, slrpnk.net has an image proxy, too.

Maybe your app has a different button to open the original post, or a button to open the Lemmy webpage for the post in your browser and then you can click the rainbow pentagon there...