Sure, you need to know something about the image you want to change. A path or filename will do if it is unique.

Here is a sample-search:

try:
    # I need to do this to make my editor happy
    import poser
except ImportError:
    poser = None

import re

def find_image(node, name="Image_Source", value="myImage.*"):
    if node is None:
        return None

    if node.Type() == poser.kNodeTypeCodeIMAGEMAP:
        test = re.compile(value)

        for inp in node.Inputs():
            v = inp.Value()
            if isinstance(v, basestring) and test.search(v):
                return node
            result = find_image(inp.InNode())
            if result is not None:
                return result
    return None


sc = poser.Scene()
        
for mat in sc.CurrentFigure().Materials():
    for shadernode in mat.ShaderTree().Nodes():
        # change value to what should be part of the 
        # path you search for
        node = find_image(shadernode, value="Mask")
        if node is not None:
            print node.Name()
            print node.InputByInternalName("Image_Source").Value()

This should give you an idea how to solve your problem.

---