Hmm, I regard myself as quite stable and not at all ill but have always found it impossible to delete images once they are in my head. I don't think about them often but they are still there and I'd always like warnings on news reports, documentaries etc so I can choose to avoid them - I'm capable of understanding the issue and taking relevant action without shock tactics.
There is something about images, with descriptions, of animal cruelty - perhaps their simplicity, there is no doubt about where power or responsibility lies - that makes them particularly likely to stick. Narrative description can have almost the same effect as I can conjour the images.
I don't know what people think they are doing by posting images of abused animals on facebook - I saw one of a dog this week. Without the story it doesn't have the same impact, I just think 'yucky picture, go away' and it only makes me think the poster is odd, not that I should give to a charity - charities themselves don't operate in such a low-grade way, they occasionally use shocking images but with well thought out text and a message. 'Can't humans be horrible, poor, poor doggy' is not one such.
I do think though, that for the issues that bother you because they are really happening in the world, you need to focus on making a difference in the world, not just to yourself. A relevant charity in this instance might be WSPA, who work on animal protection internationally.