We do not provide stock photos with a transparent background. But it is very easy to create them with Photoshop or with any other graphic software. You can use images with a white background for easier replacement of the background.
Use the keyword "isolated" to find images with white background. These images can be used much easier to get the background removed.
Most of our vector files can also be edited to have their background removed and get a transparent background. Just ask your graphic designer how to do it. Keep in mind to export all files with transparent background in the PNG Format or GIF Format.