If the images themselves are .png then you just change this:
texture_unit
{
imagename.jpg
}
to this:
texture_unit
{
imagename.png
}
or if you want to change the extension of the image itself then you will need to open the image and use "File > Save As" and then use "Change file type by extension" or similar wording.
Also make sure that the names of the images match exactly what is typed into the .material file.