This can be done via the WYSIWYG editor of ezpublish.
The WYSIWYG is used on XML block fields of your class.
There is an image upload method that does what you need. But I don't know if it's possible without the editor.
I didnt use XML blocks, and i now tried to create class that has one text line and one xml block as fields. Than i created one class in the content root, but i didnt get WYSIWYG editorm, just plain editor like notepad. How can i get WYSIWYG editor, i havent seen it yet in EZ?