[Akutni]

This is the usual structure of biliverdin – https://commons.m.wikimedia.org/wiki/File:Biliverdin3.svg#mw-jump-to-license

But I've encountered this image – https://cs.m.wikipedia.org/wiki/Soubor:Biliverdin.png
Why is the structure different?

[Babcock_Hall]

The only structural difference that I have seen so far is the difference in protonation states.  Is that what you mean?

[Borek]

No idea how to understand the X in place of double bond - I think it potentially does make the other structure an isomer of the basic structure. Not that it makes much sense, it is a product of a heme metabolism, so I would expect it to retain the basic symmetrical structure..

[Babcock_Hall]

My eye passed over that.  I have seen such an "X"  once or twice before, and I assumed that it was a double bond.  In other words, I assumed that the "X" was an anomaly of the drawing program, yet I am not 100% certain.

[sjb]

The crosses in the double bonds usually mean either it doesn't matter; or that the double bond is defined but it is not known whether it is E or Z