When you upload images to your account, we optimize them and save a few versions of your file so that different sizes are accessed and used depending on where on your site the image is used, and what device someone is viewing your site from.
Our recommended image dimensions for high definition images are:
3000 pixels wide x 1730 pixels tall for horizontal (landscape) images
1730 pixels tall for vertical (portrait) images
This is optimal for galleries, to make sure you can take advantage of full screen modes, and fill the full space in templates.
Of course there are places throughout the site that don't display this large, but if you want to use the clipboard tool to upload an image once and use it in multiple places, you'll want to have the option for the largest image possible.
We recommend you optimize your images to 72 DPI or Save for Web before uploading to optimize the loading time of the images.