@korilynneillo I've done a good amount of graphic contract design work like this and the main problem you're going to face is just not knowing how long the project will take. I've been doing it for over 10 years and I can tell pretty quickly how long something will take give or take about an hour.
Graphic design work like this is usually work for hire, so you're producing artwork that the client owns once you're done with it, so it's just hours for dollars.
What you're describing isn't completely clear because "promo images, fancy headers, etc" could be 2 or 15 images so you need specifics. What you might do is go to several kickstarter campaigns that have funded and just list out the number of assets would be included in a project as an example if they aren't 100% sure.
The way I've structured these is it's a project fee so they can control their budget, but it's based on a number hours and I'm very clear about that, so if the needs begin exceeding the original requirements (also called scope creep), you build into the contract that a revised contract will need to be signed before the additional changes can be made. Or they just say "that's OK lets stick with the original amount". Either answer is fine because you aren't getting taken advantage of or they simply pay you more for the additional work.
I'd charge between $75 and $100 an hour, calculate the hours then multiple that by 1.5. You always want to build padding in for difficult clients and OCD clients that take up a lot of time in project management. So, let's say you quote the project for a banner, info graphic, email template, facebook cover, 5 custom assets and 10 headers you know you can tackle that in 15 hours from concept to final revisions, you'd multiply that by 1.5 and quote them 22.5 hours at say $85 an hour for a total project cost of about $1900.
Make sure you include revisions so they know they will be taken care of. With the 1.5 multiplier in there, I can give them extras when I know I'm ahead of schedule or decide to stick to the original contract because they are burning time being difficult to work with. Either way I can deliver what they want and feel like I was paid fairly or I can get them outstanding service because they are being super easy to work with.
You don't want to work for clients that think this is outrageous. Any professional that is serious about their project is expecting something like this or a company that is going to quadruple that cost just for shits and giggles to see if someone accepts the bid. And any professional that knows what these services really cost and values good service at an honest rate will think highly of you for offering a straight forward and honest proposal.
Follow the final invoice up with a hand written thank you note and a $5 bottle of wine at Christmas and you've got a client for life.