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).
The value needs to be specified in KB, for example if you want to increase to 50MB, then type 51200. If you type in 0, then there is no limit to the maximum attachment size. Click OK and close the registry editor. Now you will be able to attach large files. Please note, even though you can attach any size using this hack, the actual attachment size depends on your email service provider limit.