Crop the image to a square first, then print it at the final physical size you need rather than filling the whole A4 page.

In Picasa, use Crop and choose Add Custom Aspect Ratio to make a square size such as 8x8. Export or print using that size so the photo remains square.

The key point is that a square JPEG alone isn’t enough if you print it full-page on A4—the print size must also match your frame. Print the square image at the frame’s dimensions (or slightly larger if you want to trim precisely), leaving white margins around it on the sheet.

If your software doesn’t make layout easy, another approach is to use a tool that places your square photo onto a standard-size print sheet with borders, so you can print and cut it accurately.

So the workflow is:

1. Crop to a square.
2. Set the print size to your frame size.
3. Print on A4 with margins.
4. Trim the excess paper to fit the frame.