In your switches you have an 'autogenerate accounts' option. Turning this on causes an account to automatically be created for guests who submit links, if they supply their email.

The username for the new account will be the submitter's email. The password will be emailed to them. The image being submitted will be automatically associated with the account. If the email used in the image submission matches an existing account, the link will be associated with that account and no new account will be created.

Notice that a different email from the normal registration email is sent out when autogenerating.

You'll probably want to use this in conjunction with the 'treat emails as usernames' switch.

To autogenerate on a per-link basis, you can leave the switch off but place

Autogenerate an account? <select name="sendinvite"><option value="no">no</option> <option value="yes">yes</option> </select>

in your 'suggest image' template.