Background: My work's been covered in Wired, Verge, TechCrunch, Make Magazine, IEEE Spectrum, Hackaday, Engadget, Lifehacker, Gizmodo, etc.

Send them an email.

To answer your specific questions:

A) Do i help them craft an interesting angle to the story?
Yes, the 'interesting angle' to the story should be embedded in your brief, custom taylored description of the site for that person. Don't make it obvious that you created the angle for them specifically.

B) Do i show them how it ties in with previous stories?
Yes, again embed it in your custom taylored description for that person. If possible, don't make it obvious that you came up with the tie in for them specifically. It should come across as a true component of your website, and not something that you custom taylored to hook them.

C) Do i try send pics or a link to the site? or interesting infographics or attached pdfs about site?
You want to deliver your message to their brain as seamlessly and effortlessly as possible. The email should be kepy brief by displaying just enough info to get them interested. They should be able to gauge their interest by just glanging at the email. You should also provide links or attachments for them to learn more if they're interested. Don't overclutter with those add ons though.

D) Do i ask them straight out to cover the story or do I say something like i think your readers may find value in our offering?
You can mention in a single sentence at the end that you're sending them this because you thought they might be interested in it considering their interest/previous coverage of xyz.

Further advice:
Once you've got your list of people you'd like to contact, rank them in terms of which people you would like most to cover your site, and which people you care less about. Plan on contacting the people you care least about first. This is because 1) They will be most likely to say yes, and 2) You will be learning how to tweak your email/pitch as you go, so it will be best to do the learning on the people you care least about. That way, by the time you get to the top people you'll have a well polished presentation that you're sure will work.

Again, the recipient should be able to gauge their interest at a glance. Depending on how interesting your site's concept is, you may be able to just post a screenshot or link and put a heading under it saying, "A website that does X". But make sure to put the personalized hello at the start and a sentence at the end showing that you're familiar wither the person's / blog's interests, and thought it would be relevant.

Composing an email could take a couple days because to do it effectively, you'll have to take a break of an hour or so, look back at it, make changes, take a break, look back at it, etc. Each time you take a break and look back at it you'll notice one or two things that inhibited the easy flow of your message into your brain. The breaks are necessary because they allow you to come at it with a fresh look each time, just as your 'targets' will.

If you'd like me to review and edit your draft(s) send me a message, I'd be glad to help,

all the best,

Lee