Fitting all pictures in a square canvas

The excellent free product www.irfanview.com will do this fine

It has a batch mode (B key), and the Advanced options button allows you to crop/resize to any aspect size and ratio, with many options regard how this is calculated etc

With Photoshop you can use Image Processor Pro (link - scroll half way down page) to process images that will resize your photos to a specific size while maintaining aspect ratio. You could run an action to crop (aka - reduce canvas size) but as @Rob said if you do not know where the subject is how can you batch photos and crop them? You may have to do this manually. I do not know if other programs they will achieve what you are looking for but check out Image Processor Pro as it has several good tools for batching photos.

Here how I do it for my online store

How to prepare picture for magneto: using xnconvert:

1. batch crop the white space in all the image using the source folder setting
2. re-size all the images to 500 pixel - or any size depending on the quality of your images - on the longest side
3. re-size the canvas to 500 x 500 or to the size you've chosen

Done... ;-)

If you are willing to use java, here is code I used to make images square. After this you can easily fit those in fixed size square canvas using photoshop.

BufferedImage image = ImageIO.read(IMAGEPATH);
int w = image.getWidth();
int h = image.getHeight();
BufferedImage square= new BufferedImage(Math.max(w,h),Math.max(w,h),BufferedImage.TYPE_INT_ARGB);
Graphics2D g = (Graphics2D) combined.getGraphics();
g.drawImage(image, (Math.max(w,h)-w+1)/2, (Math.max(w,h)-h+1)/2, null);
// Save as new image
String fileName=image.getName();
ImageIO.write(combined, "PNG", new File(path, fileName.replace("jpg","png")));

I would separate the landscape and portrait images and create a separate batch file for each group.

All the landscape image horizontal widths would be adjusted to the width needed, then the height would be changed to the same as the width. So if they all need to be 1000px square, then hitting Play should shorten the image width, then the canvas height would be set to 1000px height (adding white space top and bottom).

Then you would batch all the portrait images the same way, but adjusting the image height to 1000px, and the canvas width to 1000px (adding white space left and right).