In the WordPress admin click, Logo Showcase -> Add New. Enter the title of the logo. This will be for itnernal organization as it will not appear anywhere on the site.
Uploading the logo image
Even though uploaded images are automatically resized to 1200px (if larger than this size, of course), I would suggest having logo images resized to an even smaller width to save bandwidth. I would suggest keeping the images in landscape mode and have a maximum width of 350px. You can read how to resize images here. Once resized, you can upload them to Compressor.io for further optimization.
Once all optimization and resizing is done, click Set logo image from the right meta box:
Then, upload your image and set the Alt Text and Title tags.
Don’t forget to Publish!