[Suicide Junkie]

- Even single-computer, sequential turn games can be played hotseat multiplayer. AI players can be switched to human control at a moment's notice. SE4, really can't tell if you're playing alone.

- As for why it is complaining, probably a safeguard for modders who might forget to add a generic image.
As far as SE4 is concerned, anybody could be playing it, not just you. That isn't the case here, but it dosen't matter.

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If you want to avoid the complaints WITHOUT making new generic images, simply set the ALTERNATE BITMAP NAME to something that already exists, preferably something close in size to what you've added.
That will fix all the problems with using other shipsets, automatically.