Am I overreacting?

Can't see that sending this picture is anything to do with being bi-polar. I can understand as I also will not look at even photos of spiders, not watch them on television.
However, this is your sister - and people do have a very different idea of what constitutes a joke. Most definitely not worth breaking off your relationship with her. You are fortunate to have a sister. Keep the relationship going as long as you both can.

Other than having huge "ups" and "downs", I don't know much about bi-polar disorder. Is this nastiness caused by the disorder or is it just indicative of a person with a spiteful streak? I expect someone on here has professional or personal knowledge of bi-polar disorder and may be able to answer.

It's up to you to decide whether you can cope with this sort of behaviour. Even if someone may not be wholly responsible for their behaviour because of certain mental health issues, I think*FarNorth*'s comment is right - whilst you may in your own mind "make allowances", I think rude or unkind behaviour should be challenged and it be made clear that it is unacceptable.

She does sound rather immature

I'd give her the benefit of the doubt and assume it was thoughtlessness and genuinely not understanding how upset you would be, (unless she's done that type of thing before).

From all you've said, I think your sister is quite self-absorbed. Look after yourself and don't pander to her too much.

I agree with wordy

I don't think that you are over-reacting. I am afraid that her action with the picture may well be deliberate because there remains some sort of beef with you and the rest of the family, at least inside her head if nowhere else.

You are kind to want to re-establish contact with her but if I were you, I wouldn't expect much from her in terms of her behaviour. Look after yourself.

It was thoughtless but maybe she really doesn't understand how awful your phobia is. I wouldn't let it interrupt an already fragile relationship.

Perhaps she doesn't understand how much just a picture can upset you. A phobia is different to not liking something.

Could you go for de-sensitization treatment (exposure therapy) if this is affecting you so much?

It is if you have a phobia. Tell her to act her age and not her shoe size.

Just ignore it. Delete pic and never mention it again. In the scheme of things it's not a big thing. x

Rolande
If it were me, there wouldn't be another conversation.

That's just mean . I feel the same way as you about 'surprise' pictures of insects.

i really dislike people who do things like that and cloak it as a joke.

It's not big and it's not clever . I'd tell her quite calmly that it wasn't funny. Then carry on as normal. If she does it again then that's a bigger conversation.

not a nice thing for her to do to you , she is warped !