You can choose different image sizes when you send an email. You could choose smaller images to save space or larger images to preserve the image quality.

When you are viewing the send email screen, tap on the "+" button beside the attachment label, or the paper clip button in the top of the screen. This will show the cloud server, file and image chooser. Scroll down and select an image album. When you tap to select images, you will see a size menu button in the bottom of the choose image screen. You can use the size menu to choose the size of the attachment.
You can also choose the format of the attachment eg. png, jpg or original format. Using jpg will make usually make the image file smaller.

The order that you select the images will be the order that they are attached to the email.
You can delete and rename images in the attachments list when you are editing the new email. Tap to check images and use the action menu beside the attachment icons.

You can resize an image after you insert it into the email message content. On an iPhone or iPad you can pinch the image with two fingers to resize it. Double tap on the image to change the image file size.
On a Mac, you can right click on an image in the message to change the display size as well as the file size.