rightsproblem after reinstallation

Last week my wordpress website was infected.

Only the code was infected, the database was still okay.
I have removed all files of the website and placed my backup of the files.

The website works properly for the end users now.

However, there is a rights problem that occurs when placing new media files with the dashboard and by using the NextGen gallery. To be able to add mediafiles with the dashboard I set the rights of userfiles/uploads and wp-content/gallery temporary to 777. Add media files succeed and creating new nextgen galleries also. It now appears that newly added files has the owner/group 99 99. So that’s the problem. The wordpressuser is now owner/group 99 99. For the ‘reinstallation ‘ this was another owner/group.

How do I fix this?