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).
Enter the desired attachment size limit in KB under Value Data:. To set a size limit of 25 MB, for example, use 25600. The default value (with MaximumAttachmentSize not present) is 20 MB or 20480. For no attachment file size limit, enter 0. Practically all mail servers do have a size limit, though, so 0 is not recommended; you would invariably get large messages back as undeliverable after an often long and fruitless uploading process. Ideally, the limit corresponds to your mail server's limit.