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).
1 If there was no limit on the size of attachments, then users would send huge files causing massive storage issues for the back-end mail store.  Performance. Email systems aren't optimized for transferring large files. Client systems often download messages to the local computer.