You are not the asshole. I mean, the fact that he didn't respond immediately is not an asshole move if he was busy or something, but if he ignored it, he is an asshole.
Sending the sh pic is not the best move, but I get why you did it and I don't think you are an asshole for doing it, just reconsider it the next time, even if he is suicidal.
I do think that opening up to someone who is not suicidal and etc is harder, but if he is your friend, I am sure he is going to make his best efforts to try to understand you and, even if he doesn't understand, he is going to care about you and do his best to be there for you.
I have had suicidal friends who didn't care; non suicidal friends who didn't care; now I just maintain in my life people who do care about me, whether they understand me or not. I have friends who have never had intimate contact with suicidal people before, and at first they were a little shocked with me, but with time they became the best friends I could possibly have.
So choose the people you are going to have in your life; trust me, they are not going to necessarily understand what you go through and they might not be suicidal or damaged in some way, but they will care. That's what real friends do. Not the easiest people to find, but it's possible and it's worth it.
And "keep it to yourself"? Sorry, but fuck him. He shouldn't even have talked about the picture, he should have showed you he cares.
Anyways, don't blame yourself for shitty people's actions.
And if you need to vent, you can pm me anytime.