Thanks Kautuk for the reply. I already tried that and was able to add it but ran into some issues so we decided that we will use the image plugin from classic UI instead of touch for now so i want to make it work without adding customization for image plugin. Everything is working fine but the only issue is with the value in the drop down not resolving to a value and displaying the label instead.
The label is defined in /libs/cq/gui/components/authoring/dialog/richtext/clientlibs/richtext/js/RTE.GraniteI18nProvider.js but its not picking it up. It looks like its using another js file //libs/cq/gui/components/authoring/editors/clientlibs/core/js/editors/RTE.UiTouchI18nProvider.js instead of the Granite one.
Is there any other solution on how to fix this or my only option is to add the customized code to make it work in touch ui.
Thanks
Santhosh