The caption of the upload button is a (translatable) system text, which you can define in the Modeler (in category "Data view"). However, this will change the caption for all Image uploaders and File managers. If you only want to change the caption for a particular Image uploader, I suggest you file a feature request.
I think it will be difficult, since the label of the upload button is not a property (which you will already have noticed).
A possible hack may be
I think it is not worth trying above, and guess you'll need a feature request to make it configurable