So here is a great tool I have been using, it's called Canva.
Canva let's you design cards, business cards, media headers, email headers, and of course Twitter Cards. It's costs $1.00 per card but there are also free cards depending on the design. I've been using Canva, for my Twitter posts, to attract more attention in the live stream. These posts stand out a little more and catch users attention.
I used it to create the header on my website, SMR Website Design, and it looks great. I also rotate a few Twitter cards throughout my daily posting as it introduces a fresh look for viewers.
The fact that they make the cards for a Twitter feed or Twitter header, makes it extremely easy for you to design one that fits perfectly for the feed.
They also offer Facebook and Google+ headers as well.