Hi Carrie,
MapSeries is doing its own image processing if the image uploaded is larger than 1500px wide or 10Mb in file size. It looks like yours is about 5000px wide and 70Mb in size. Apologies that it's getting so pixelated in the process.
To avoid MapSeries trying to downsize your image, try resizing it yourself on your computer to 1500px wide before you upload. Also make sure the resulting file size is less than 10Mb. Additionally, if you're just using this image for the small logo in the top corner of the page, you may want to resize it on your own computer much smaller -- say, 200px wide.
Thanks!
Alison