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).
To get around attachment file size limits, you really have three main options: 1) Select an email provider that allows for large attachments 2) Use a cloud storage solution to create links to files that can be put into an email or 3) Use a file transfer service, many of which are free or freemium. Email providers.