I have what I consider a nasty habit of putting my primary keywords in the titles, descriptions, and alt tags of my images in addition to having them in across my articles (title, headers, and body). While I think of it as over-optimisation (it's really just the way I organise the images when I upload them), I have yet to notice any repercussions from doing this on any of the sites I've done SEO on (this still doesn't mean it's right).
To answer your other question, using your primary keyword 7 times in a 2000 word article means your keyword density is 0.35. While there really is no exact keyword density you should aim for, I would personally use the primary keyword a few more times along with a few LSI variants. Like renzuken said, test things out on your site and find out what works.