Tl;dr:
This was probably a failed attempt at porting. Search the other channels of the same package for newer versions.If it is new, there is likely a nightly build, although I doubt you found this on stable. Check Flatpak for the same package. Chances are you will find it.
Conclusion:
It looks to me like you have a broken package? It is failing because it cannot find a shared library that it needs.
So technically, the answer is:
you need the file libportal.so
however..
1.) The fact that it is not included with eog or wrapped in package in apt is strange
2.) After a bit of research, it looks like the library is part of the flatpak api. In fact, the flatpak sandbox abstraction is sometimes called the "portal".
I am not a flatpak developer by any stretch, but I know enough to know that the sandbox implementations are way different to share apis.
My guess is that the package was recently ported from flatpak, and there is a discrepancy somewhere in the metadata for the package.
The gook news is that it is most likely available on Flatpak! Although if the app needs any significant permissions - i.e. access to the OS's devices or other global resources - to run, you will soon know what those differences I mentioned all too well.
