When you upload a file from a form to a server the file goes from the client's machine to the server and is accumulated in a temp file on the server. The server will create this temp file in the temp directory on the server. From here it is then moved into the location you specify on the live server and named accordingly.
If you do not have proper permission on the temp folder or the folder the file is being moved into you will receive errors like this. To resolve these errors you just need to make sure you get the correct permissions for these locations. Most of the time the problem is that you don't have proper permission on the temp folder and therefore cannot get the file moved to the live folder. Make sure that you have read and write access on this temp directory, if you do not have access to this directory to make these changes your host or admin will need to do this for you.