In the Creature Creator, click on the Settings button.
Select the “Graphic Settings” tab and adjust all options to High. Also adjust Screen Size to the maximum size.
Select the "Capture Settings" tab and select Large Picture Size.
In the Creature Creator edit mode, after you've finished building and painting your creature, click on the "Test Drive" button at the top of the screen.
Select the backgrounds option (it looks like a tree) and select the blank background option (the plain black one).
Zoom in to your creature to make it as big as possible by using the zoom buttons at the bottom of the screen.
Take a picture of your creature by clicking the camera button.
Find the picture on your computer in the folder \My Documents\MySporeCreations\Pictures. Each picture is saved as two separate versions, small and full. Be sure you can find the full version (_full at the end of the filename). That's the one you want to use to create your Custom Creature Product.
Make sure your picture looks good. You want your creature to be large and centered in the frame, and not cut off.