Unable To Create Thumbnail

Hello all,there is a trobule when i delete all the thumbnail,and then i visit the admin panel.it shows this error

ErrorUnable to create thumbnail for file "/www/wwwroot/xxxx/images/logos/4/logo.png": Get operation failed

Can anybody help?

thank you very much.

If you deleted the images/thumbnails directory manually, start from checking if this directory was created again and it has correct permissions