Photos these days can are often in the Megabyte range which makes it difficulty to transfer them via email or other means. Email is special as it has a it is usually limited in the amount of Megabytes you can send per email. The limit is usually in the 15 to 25 Megabyte range, with companies like Microsoft working their way around the limit by hosting the attachments in the cloud and replacing the attachments with links to the cloud hosted files instead.
Some users help themselves by running their photos through image optimizers like Riot, VSO Image Resizer, or a dozen of additional image resizers optimized for that task.
The Thunderbird extension Shrunked Image Resizer handles the task automatically whenever you send images as attachments. The extension displays a configuration dialog whenever you do that allows you to select a desired maximum resolution for each photo, as well as the quality of the converted images. You can furthermore define a minimum file size in the extension options. All images below that size are not converted at all, while all greater than the selected size are.
Once you have made your selection you will see a progress bar coming up that indicates the conversion progress. The process itself should not take long, and you will notice that large images are reduced significantly in size by it.
There is no preview of the output image which is one of the things that could be improved by the author, as here is no way currently to check the quality of the converted images before they are send out. You could send the first batch of images to your own address to verify the quality of the conversion before sending them to other recipients.
Shrunked Image Resizer regardless of that is a useful extension for Thunderbird users who often encounter issues when sending photos as attachments in the email client.