There has been quite a few posts regarding Lemmy apps for iOS, and their installation process might not be straightforward since they're PWA or beta. Here's a list of apps mentioned and their links:
⭐ Released
Voyager (formerly wefwef) (📱PWA)
To install, open link in browser and follow instructions
Just downloaded Thunder, but is seems to have the same issue on iPad as Memmy, where image/web previews are reduced down to a short and wide slit of the image. wefwef is the best for iPad that I’ve used so far.
Edit: scrap that, there is a setting on Thunder to view full height images.