If anyone else is running on Plesk and having the image uploads problem (and CHMODing to 777 is not helping or anything else), the following will probably help you... it solved it for me.
In WP admin, go to Settings/Media page and make sure the text box after "Store uploads in this folder" is blank... that causes WP to use the default path, which is "wp-content/uploads". If you know that your upload path is different than the default, then put it there, but the usual is to leave it blank.
Now, do the following in your Plesk File Manager:
Rename your "uploads" folder to something else, like "uploads_old". Then create a new folder and name it "uploads".
Now, go into your WP admin and try uploading an image. It should work. If not, temporarily chmod your new "uploads" folder to 777, then upload. After you get it working, you can chmod it back to 755. (I'm not sure why this worked for me, but it did. It may have just been a lag in the server resetting after creating the new folder and I really didn't have to chmod to 777).
Now you can upload any other folders and files to your new "upload" folder, but be careful not to overwrite and new folders/files that you just created.
Hope this helps you Plesk users out there or anyone else using a similar system.