.png == portable network graphic, a 2D pixel image
stl == comes from an application “Stereolithography CAD” and was used as the file extension for 3D data from that application
Some previous tutorials which may help:
(note that importing a pixel image is much the same as an STL, just no rotation option)
If you’ll upload your file we can walk through this with you here — just let us know stock thickness and how you want things to cut.