Call the Venue or Promoter. As soon as you know you will need to cancel the gig, call the venue and/or the promoter, contract or not. Obviously, this is important information for them to have, but this is also your chance to mitigate the fallout from the cancellation.
Instead, you change the size when you are sharing the photo. For example, if you choose to email the photo, the Mail program will ask you to choose the size of the image (I believe it is small, medium, large, original).
When you can't reduce image dimensions or resolution to make your files more portable, you may need to resort to other alternatives to enable big bitmapped images to piggyback onto email transmissions without your messages bouncing back to you as undeliverable.