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 do this you will want to make use of clean code, optimize your images and use inline CSS correctly to keep the file size down. The design of your email blast should be no wider than 600 pixels, so that the email can be clearly read in smaller screens or in the outlook preview panel. Most good email service providers will have optimized email templates available for use.