Undefined property in a Views PHP field
Users who have not uploaded a picture (also the anonymous user) will not have any object set for $account-&gt;picture. You just need to check that there is an object available:

$uri = isset($account-&gt;picture-&gt;uri) ? $account-&gt;picture-&gt;uri : NULL;


image_style_url() will return a bogus URI when passed NULL. You'll want to handle this somehow.

Note:
The user picture is already available as a field, you don't need PHP to get it.

In views 3, you must first add the "Content: Author" relationship before the User fields (including one for the user picture) will show up.