Add text to slider image/featured image

Hello there,

I am currently using Name: Portafolio Version: 2.11 and running WordPress 4.6.1
I am trying to add text on top of a slider image. The Portafolio template has already a slider embedded in it but I am unable to find a way to add text over an image here. There is a caption option however the caption will not show text on top of the image on the slider page.
I have tried downloading widgets for photo editing but these don’t do the job as they seem to work in this template’s slider section, only in posts and pages.

Is there anyone out there who can help? I have contacted the developer of the Portafolio template, their suggestion was: Adding text would be easiest via a photo editor on the image, otherwise you need to edit content-slides.php But I am unable to find a photo editor that enables me to add text to a slider image, i.e.: something that is not embedded within a post or a page.

I hope I have provided enough information. Thank you in advance for your help.

Dawn