Snippets are not editable by the end user. If you want to have the end user edit them you need to make them either modules with editable areas (such as rich text) or add variables for the editable elements (such as images, colors, or strings of text).
With the addition of module-level variables now added, I would explore setting up a template with the new Email Editor 2.0. You can lock down styles and crucial parts of code in the email while still allowing your marketing team to customize content.
Yes, upgrading to v2 is much better, but it can require some work, and you need to be cautious with old v1 templates.
See Upgrading to the new email editor 2.0: a recommended migration path
-Greg