Customise the "Choose From Designs" and "Add New Image" text

Hi there

I am trying to change the wording of the options currently showing on my product designer.

See screenshot: 

So I want to change it to say Upload Your Own Image or whatever. Is there a way that I can customise this in the back-end interface? If not, can someone please direct me to the file that I should manually go and edit?  

Thanks :)