Membership plugin or a simpler solution for this?

I am developing a site for a client that needs a membership element for the purpose of keeping some content from non-certified industry professionals (specifically prices). Those who are certified in their field would apply to register for the site. The approval process is manual and requires that those who request registration, submit an image/picture proving they are certified.

MailChimp is used for a mailing list. They can be used to make a form for this manual registration process, but they don't have an option to attach an image to the form. MailChimp did tell me that WuFoo could be a help to this. BUT If I have to use a membership software/plugin, will all of them need me to use their own registration form (which would make sense to me) in order to have a level of user that some information is kept from.

Do I need a full-fledged membership software/plugin or is there a simpler answer?

I've looked at s2Member and it seems frightfully complicated. If this is so simple that I don't need to purchase a software, that would be best. Any suggestions?

Thank you for any help you can offer. I need an answer as soon as possible.