Photo backgrounds can now finally be included in custom cards!
As you can see, the background has been extracted from the original image:
Step 1: Load the image you would like to use into GIMP
Step 2: Create a new layer mask
Right click on the layer with your photo and choose "Add Layer Mask".
The mask is like a layer that filters another layer. The original image will be visible wherever the layer mask is white, and transparent everywhere the layer mask is black.
Ensure that you are currently editing the mask by clicking on it in the layer panel.
Step 3: Paint the background black
Select the paintbrush tool and make sure you are painting the color black.
Once you have painted the entire background black, only the player will remain. You will see a white silhouette of the player on the layer mask thumbnail.
Step 4: Save the file as a PNG
Go to File -> Export As and save your file as "Player-Name.png". *Make sure you enable the "Save color values from transparent pixels" option!*
Step 5: Place your photo in the card generator
Add your photo to the card generator as usual, but enable the "Use photo background" option.
Position your photo and ensure that all four corners of the card are covered by the background.
Step 6: Complete
This is just one of many ways to achieve this using GIMP. It is also possible with Photoshop, and with a bit of effort you could also include different backgrounds in your photos. The most important thing is to avoid the erase tool as it can change pixels to black rather than making them transparent.
Have fun creating cards!