There are a few different reasons for this error to occur. When I update the flash player to 10.2.152.26 version, and WordPress 3.1 flash uploader shows IO error . I think other WordPress users have encountered this issue. In the following simple solution, I’ll show you what I did to address the problem and get my WordPress Image uploads to work.
Simplest Solution 1:
There is one Simplest Solution: Do not use Flash uploader. Use Browser Uploader. So I tried that instead of the original Flash upload thing and I was able to upload my pictures. Hope this works for anyone else who’s getting this error!
There are more solution for this error. You can find them on following Links:
IO Error Uploading Images WordPress:
He explain how to Uninstall Flash Player Plugin to resolve this issue. I tried this one and it is very helpful.
WordPress IO Error and How to Fix It:
This explanation is not for MultiSite WordPress installation.
WordPress 2.5 Image Upload Error [WordPress Fix]
You have to Insert the script into .htaccess file.