Depending on where you plan to use your image files we recommend different sizes:
Image Select - we recommend image files of no larger than 200 x 200px.
Header Images - we recommend images files up to 750px wide and 200px tall.
Banner Header Images - If you would like your header image to display like a banner that stretches across the entire page, we recommend image files 1000px wide and 200px tall. To accommodate this image size, change your Page Width (available under Style > Layout > Widths/Sizing) to 100%.
Background Images - For background images you wish to use as a cover screen, we recommend, as a rough guideline, 1920 x 1080px or smaller with the same height-width ratio (1.777 to 1). For more info on full-screen background images check out this awesome article: http://www.webmalama.com/the-best-full-screen-background-image-sizes-for-web-design/
Options for Resizing Your Images
When uploading images to the file library you have the option to scale your image using pixel width or a percent. There are also various tools online to resize images e.g. http://www.picresize.com/.
It is important to note that resizing images may negatively affect the quality of the image.