Just a warning: if you're posting your pictures on tumblr, they'll be forcibly downsized to a maximum width of 500 pixels (regardless of how big you originally made it). That's the size it'll stay, so when you add the image into your fic, it'll show as 500 pixels wide.
This is a problem because normal cover art is at least 600 pixels wide, and the banners (which take up most of the screen) are 1000 pixels wide. It looks really, really awkward to have the 500 pixel width image (it looks tiny), which is why I stopped hosting my images on tumblr and created an imgur account.
Imgur won't change the size of your images.