Clarkii can use an xml file that stores all layer and graphical elements information of an image. You can create a 'template', e.g. a banner with 'type your companyname here'. When the customer opens that template in Online Image Editor he can replace the text with his own and preserving all the other layout elements. He then can upload the image to server (as jpg, png or gif).
A template can consist of an unlimited amount of elements or layers, layers can be 'locked' to prevent distorting the actual design. A user will be able to change the text in the template, but cannot move the text to a different location in the design.