Files can be downloaded in two different ways.
From browse and search results:
You can download a high resolution version of the image by clicking on the download button and then choosing the 'Download High-Res' option.
To download an FPO, choose that option if it's available. What is an FPO?
From a collection:
You can download a group of images at one time. First you need to add the images you would like to download into a collection. Then in the collection you can click ‘Download Archive of Files’ which will create a zip file with all the images that are present in the basket. Give the zip file a name, then choose the format, then choose whether you want to include an XMP Export. An XMP Export will export the file’s metadata as a separate text file. Once you have done that, click download basket. You can do the same for ‘Download Archive of FPOs’.