Sorry but I am not going to 'subscribe' to anything to download your template. (plus I hate smart links 
)
In general terms these templates (made for PS) have many layers, not all effects / layer types work in Gimp but there should be a layer somewhere in which you paste an image. Post a screenshot of what you have showing the layers dock. Remember, there might be layer groups that need expanding to show the layer for the image.
At the simplest it might look like this.
A base layer
An image of a phone with transparency
and an in-between a layer usually with a layer mask for your image.
A quick 40 second demo of pasting an image in. https://i.imgur.com/fAaLT6f.mp4
)In general terms these templates (made for PS) have many layers, not all effects / layer types work in Gimp but there should be a layer somewhere in which you paste an image. Post a screenshot of what you have showing the layers dock. Remember, there might be layer groups that need expanding to show the layer for the image.
At the simplest it might look like this.
A base layer
An image of a phone with transparency
and an in-between a layer usually with a layer mask for your image.
A quick 40 second demo of pasting an image in. https://i.imgur.com/fAaLT6f.mp4