Once in a while a customer will ask me this question — believing they should upload the image size they require for their content to the WordPress Media Library. For example, if they require a 300x200px image for their content, then they upload a 300x200px image to the Media library. What they don't realize is that WordPress isn't meant to be used this way, and they're actually breaking some WordPress features by doing this — not to mention that NextGEN Facebook will probably reject the image for being too small for Facebook and Google. :-)
WordPress and several 3rd party plugins provide different image sizes based on the resolution of the viewing device. For example, a 300x200px image in your content might look blurry on some high resolution screens (almost all current mobile phones, tablets, and laptops) because the browser must "upscale" the image to 450x300px or 600x400px in order to fill a 300x200px space on these high resolution screens. WordPress includes additional image markup in the webpage to provide alternative sizes (300x200px, 450x300px, and 600x400px for example), which allows the browser to choose the appropriate image based on the screen resolution. If you upload a 300x200px image to the Media library, WordPress will not be able to offer these additional image sizes, and NextGEN Facebook will not be able to use this image for most social sites and search engines (which have a variety of minimum size requirements).
So, what should you do if you want a 300x200px image in your content?
That's what WordPress image sizes are for. ;-)
In the WordPress Settings > Media page, you can define sizes for a variety of different image sizes, like thumbnail, medium, medium-large, large, etc. Your theme and other plugins may also define additional image sizes, and some plugins allow you to define custom sizes as well. When you need to add an image to your content, you select the correct image from the Media library and choose the image size you require. You should never use the full size image in your content. If you need a 300x200px image, for example, you should have defined a 300x200px image size in the WordPress settings (cropped or uncropped, depending on your requirements) and then select that image size.
So, what image size should you upload to the Media library?
That's easy — the largest size you can. ;-) Currently, Facebook uses 600x315px images for shared links, which means providing Facebook with 1200x630px images for high resolution devices. So at a minimum, you should be uploading 1200px wide images, but hopefully even larger to future proof your images for evolving requirements — both Facebook and Google may require larger images in the near future.
Because so many websites have used small images in the past, NextGEN Facebook provides 600x315px images to Facebook by default. If your images are large enough, you should change this to 1200x630px in the WordPress Settings > Social and Search Image Dimensions page. See the NextGEN Facebook Setup Guide for more information on image sizes.