test png files for Seam Carver
rename var for clarity
use ternary operators and sum comprehension
DRY out code with tuple unpacking
Refactor print seam functions to combine them

... to better highlight the small differences between the two functions
Use docstrings (""") instead of strings

You have well-documented functions here: make the docs accessible by formatting them with triply-quoted strings.  (See, eg. my_function.func_doc).
Refactor seam energy function
use shiny new function
Remove getter

It's not generally pythonic to have a "getter" like this - you usually want to just access the object's attributes directly.  It looks like you're signalling to the end user that the width and height aren't part of the object's public API.  I think it's more clean to skip this (and in fact, I'd change the names from _width and _height to width and height).  Others may disagree :)