Image Sizing Tests
Depending on the size of the image that you upload and the settings that you have entered into your media settings page, there are generally four pre-defined image sizes available to you to insert into your page content: original (the file you upload), large, medium, and thumbnail.
The test image that’s been uploaded here is originally 5760 × 3840 pixels.
While uploading an original size image to your site is fine, you should avoid inserting the original size image into your post, to avoid slow load times for your website’s visitors.
We generally recommend that you set your thumbnail image sizing to 192 x 192 pixels. We recommend this value because it’s nearly 25% of your site’s content width (770 pixels) with the presence of a primary sidebar.
If you must optimize the image for Retina devices, then we recommend setting thumbnails at 384 x 384 pixels and manually downsizing them within your post to appear as thumbnails.
Medium images are recommended to have a 385 pixel width maximum, with no limit (0) for height. 385 pixels is half the width of your website when a primary sidebar is active. If you must optimize your website for Retina devices, then we recommend setting your medium images to a maximum width of 770 pixels and downsizing them from within your visual or text editor accordingly.
Large images should be set to a width of 770 pixels (1540 for Retina-optimized websites, with manual downsizing). This value is the maximum content width allowed to your website when a primary sidebar is active.
As mentioned, uploading any size images to your server is okay, especially in the case where you would like your visitors to have access to your full resolution files. But when it comes to inserting images directly into your content editor, take care with making sure that the images you insert and use are not the culprits of slowly loading websites for your visitors, which can lead to lost traffic and business revenue.
If you’re confused about which values are best for you, we suggest going with our recommended settings; we’ve tested those thoroughly and know that they work well under a variety of conditions, on all devices.