Need to add figure to new background

My job doesn't require me to do much more than minor tweaks to photos, so this is beyond my level of expertise.

One of my students sculpted a dragon. I have a photo with the original background deleted, leaving a plain white background around the dragon. I can't remember how to turn it into a PNG, if that's what I need.

I want to add that dragon atop a background of fire. The background seems much larger than the dragon, in terms of pixel height.

I did look for a tutorial, but what I found requires more modern versions of the software than X5.

Can anyone guide me through this? Thank you, and have a wonderful day.