When you start to download a Bigstock image, you should be prompted to "Save" or "Open" the file. Choose "Save" and pay attention to where your browser saves your downloads.
You can use your browser's "Save as" command to save the image, or click on the image with your right mouse key (or control-click on a Mac) and choose the "Save image as" option. You may also be able to click and drag the image from the browser to your desktop, where it will save as an image file.
Try searching your hard drive for any file containing "bigstock" in its name. Often, files will download to your desktop or to a folder on your hard drive called My Downloads or Downloads, which may be within your My Documents folder.
Check the file extension. A file that ends in ".php" may need to be renamed to end in ".jpg." If you haven't done so already, set your computer's folder options to show file extensions, and rename the file.
Even if an image file won't open when you double-click on it, you may be able to open it successfully from your image program.
This is usually the result of a download error. Try downloading the image again. You will not be charged again if you re-download an image within 180 days of your first download attempt.