NicEdit is a WYSIWYG editor for websites, which can substitute "textareas" in WordPress.
It's very easy to apply this editor. You can just copy the code found at NicEdit's web-site at http://nicedit.com/ and paste it into a page, which will change "textareas" to "NicEdit."
I thought the code above will apply to textareas found in a specific page which contains this script, but all "textareas" will be replaced by NicEdit. If you use the following script (for the second script above), only textareas with ID 'myNicEditor' will be changed to NicEdit.
I changed 'myNicEditor' to 'comment' in order to use NicEdit in my WordPress comments system. The features are limited compared than other editors for HTML such as Ckeditor but it enables users to upload images. (Please refer to this web-page for additional text and HTML editor which can be used in WordPress.)