First, understand that high resolution screenshots can be very large - and may be larger than the maximum attachment size that many mail servers will be able to process. As such, the attached image may be automatically reduced to a lower-resolution image.
Having attached an image to a draft email, you have some control over the attachment size. To change the size/resolution, at the right of the cc/bcc field, you may see an "images" button; tap this button to see and select from the available file-size options.
When choosing a larger file-size, always consider that mail servers may reject larger files. For large files, you might be best advised to share a link to image that you have saved to a cloud service. Apple provides a useful facility for this very purpose - known as Mail Drop:
Add an email attachment in Mail on iCloud.com – Apple Support
Mail Drop limits - Apple Support