This script cycles through files in a directory (hard coded) and converts them to thumbnail png's.

PS-files-to-thumbs-png.jsx

#target photoshop  
  
// This script cycles through files in a directory (hard coded) and converts them to thumbnail png's.
// Only currently works on portrait orientation images.
  
app.bringToFront();  
app.preferences.rulerUnits = Units.PIXELS;
// A hard coded path to a directory 'pc style'  
var folderToProcess = 'h:\\source\\file';
var processFolder = Folder(folderToProcess);  
// Use folder object get files function with mask 'a reg ex'  
var fileList = processFolder.getFiles(/\.(jpg|tif|psd|eps|png)$/i);  
// Loop through files  
for (var i = 0; i < fileList.length; i++) {  
     // Only process the returned file objects  
     // The filter 'should' have missed out any folder objects   
     if (fileList[i] instanceof File) {  
          open(fileList[i]);  
          
           var srcDoc = activeDocument;
          
           var fileName = srcDoc.name.replace(".psd",""); 
           
           //300 px
          srcDoc.resizeImage(300,null,72,ResampleMethod.BICUBIC);
            //make canvas square
          srcDoc.resizeCanvas(300,300); 
          var fileNamePath = folderToProcess+"\\thumbs\\" + fileName+"-300"+".png";
          SavePNG(fileNamePath); 
          
          //100 px square
          srcDoc.resizeImage(100,100,72,ResampleMethod.BICUBIC);
          fileNamePath = folderToProcess+"\\thumbs\\" + fileName+"-100"+".png";
          SavePNG(fileNamePath); 
          
          //50 px square
          srcDoc.resizeImage(50,50,72,ResampleMethod.BICUBIC);
          fileNamePath = folderToProcess+"\\thumbs\\" + fileName+"-50"+".png";
          SavePNG(fileNamePath); 
          
          //Close without saving
          srcDoc.close(SaveOptions.DONOTSAVECHANGES);
     }  
}  




function SavePNG(saveFile){ 
  var pngOpts = new ExportOptionsSaveForWeb; 
  pngOpts.format = SaveDocumentType.PNG;
  pngOpts.PNG8 = false;  
  pngOpts.transparency = true;  
  pngOpts.interlaced = false;  
  pngOpts.quality = 100; 
  activeDocument.exportDocument(new File(saveFile),ExportType.SAVEFORWEB,pngOpts);  
}