Unable to run variable with PNG

[Lakeshore Sports5E7C]

I have 93 headshots where the subject is removed from the background and saved as PNG. I am trying to use the variable function to combine a background image with my headshots. At first I found that unless I converted the PNGs to a smart object that the variable function did give me the option to define the layer as a pixel replacement. So, I ran an action to convert all my headshots into smart objects then I tried to create a variable again. However, now I cannot define a variable unless I rasterize the layer?I cannot seem to get the settings right to combine these files using variables. Any idea why or how to go about this?

[JJMack]

Though Smart object look and act like image pixel layers they are not. Smart objects pixels are hardened and can not be changed with Photoshop tools.  In a Photoshop data driven graphics template the only control you have for a smart object  layer as  variable is the Layers visibility. Only normals pixels layer and text layers variable can be replaced other layer types visibility can be set on and off. You can Place all your portrait png with transparent backgrounds into  Photo collages template that has a replacement background,  Your PNG image will be smart object layers in the populated PSD template. You can batch populate a collage template....

[Lakeshore Sports5E7C]

Thank you for the prompt reply. I did just dive into this and try and sort out the information you gave me, but alas, I am too much of a novice to really understand it? I'll keep at er though.

[Stephen_A_Marsh]

Is it only one static background with each separate headshot?

Or a different background for each headshot?

Variables existed long before smart objects, so there shouldn't be any dependencies and I would stay away from smart objecdts if possible.

[Lakeshore Sports5E7C]

It's one static background and hundereds of pngs. 

[Stephen_A_Marsh]

Then you don't need variables, an action or script can batch process the images, inserting the static image behind the transparent image.

[JJMack]

Create a simple Collage or Mockup template  then batch populate your template with a Photoshop Script.   Or create a Data Driven Graphics template a PSD file with  Background layer and  normal Image layer (not a smart object layer) a normal raster pixel layer.  Define it as a variable and create your CSV file that has a single column  for  the Image variable header and the rows are the full file paths of your png files.   Only Normal Pixel layers and Text layers can be replaced in Data driven graphics templates.