I'm looking for some nice ways to visualize whether a user is a full member or a regular member. Assume a Facebook-style website with full membership option.
The profiles will be displayed as blocks of 80x80 with image and name in a box below.
Why do you need to display this information on a list of profiles?
Who is interested in this information?
Is there any actual benefit to other users?
For example, knowing someone is paid up LinkedIn member as opposed to someone who's just using a free account doesn't really affect me as another LinkedIn user. It doesn't change the way I can interact with that user. So in this scenario there is no utility to having that information displayed at all.
However, for a system administrator or marketing manager for the company running the site then having that information could well have utility and would need to be displayed to those users. In this case a simple "+" icon or the word "Pro"/"Full" overlaid in one corner of the image would work, as would changing the background colour of the user name perhaps.
If the goal is to convert free members to paid members then that's certainly a valid reason for differentiating the users. In this case, however you display it, some way of highlighting the benefits would be useful - this could be a tooltip or a link to the "become a full member and gain these rewards page". This could be from the icon I mentioned earlier.
Looking at this slightly differently on Stack Exchange where a "full" member is one that participates in the site posting questions and answers rather than just reading the content having their reputation score and badge totals next to their names is useful to me as user. It gives me clues as to how knowledgeable and helpful this person is, it also indicates whether I'm likely to find them in chat etc.
To sum up - work out your use cases and why this information needs to be displayed and to who. Once you have this the "how" might become clearer - or at least you can ask a more focused question.
A non obtrusive answer would be to use different color borders around the 80x80 image. This works as long as your states (full/non-full) are few in number.
You can annotate profiles to show different membership status in a few ways (in order of what I would try were I in this situation):
Think about what you want to achieve by displaying this extra information about your users.
Offering the information could be useful; it will potentially affect your site's internal culture by increasing your users' exposure to paid membership options - and the fact that paid membership exists.
Considering this further, one main benefit to providing 'pro' or 'full' status to paid members is the chance of increased conversion.
For this reason I would suggest:
An example of this methodology in action can be seen via Flickr: