When uploading images from a local workstation to Prism Central, the destination cluster is determined based on the alphabetical order of the cluster names. Specifically, the first available cluster in alphabetical order is selected as the destination for the image upload.
This behavior is documented in the Prism Central Admin Guide, which states that when you upload an image to Prism Central, "the image is stored in the first cluster that has sufficient space and starts with the letter that appears first in the image name."