As you question is regrading Web Wiz Forums and not the standalone Web Wiz RTE I have moved this post to the Web Wiz Forums support forums.
The most common reasons for errors when uploading images are:
1. The upload component is not installed on the server. Run the 'Server Requirements Test' tool from the forums admin area to check what upload components are installed on the server.
2. Write permissions have not been enabled on the forums 'uploads' folder. You would need to either set the write permissions directly on the server or if using a shared web host they should provide a way to set write permissions through their hosting control panel.