If you’re using Evolution, try: [*alias*]
For Revolution: [[*alias]]
But if anyone changes the alias or page name then you could run into problems as the image filenames are hard coded.
I would consider using an image
template variable instead. You can then load a graphic for each page using the manager which will stay linked even if the url changes. The end user will be able to change it easily without needing a web developer too
Also, to keep it SEO friendly you can specify
alt="[[*pagetitle]]" in your image tag which will give you a lot more value than the image filename.