You are an idiot if you think a basic shape == racist!
The swastika is a basic shape also. If Trump had included that in a tweet, would you defend that as well?
The fact is, certain shapes can and do have connotations, especially in the context of politics. Trump probably didn't realize the connotation carried by that shape (placed in front of a pile of money, no less!) when he forwarded the image, because he's so politically naive; but the symbolism wasn't lost on the public.
Second, the "neo-nazi site" was 8chan/pol. It wasn't Stormfront. It was a fucking image board on /pol.
Oh. Well, I guess that's okay then.
The image was created by @FishBoneHead1, who is quite clearly an anti-Semite. You can play lawyer-ball and claim that it's possible that he just chose that shape at random, but it doesn't pass the smell test, and Trump should have known better than to repost it.