Error 425 No Space Left on Device

Ok, so I am working on a website for my employer and I am not very computer Savy, and I have learned most of FTP by trial and Error but I just got the message 425, no space left on device. The simplist thing would be to erase older files, but the website contains over 6000 products with 2 images each and I am afraid that by eraseing older files those pictures will no longer show up on the website. Am I correct in this assumption? Is there anything I can do? There is still at least 1000+ images to go on the site. What can I do?