Many of the online sites will allow you to upload the photo from your computer, resize it, and download it. Easy. If you make the photos around 1000px wide, and save them as JPG, they will easily be small enough to upload to your original post in this thread.
If you have the photos on your phone, there are easily hundreds of apps for iOS or Android that will allow you to take a photo and resize it in seconds.
Also, for the benefit of any potential buyers, it would be helpful if you included the closest USA sizes next to your British sizes.