You should see the default avatar when viewing their profile. If you're wanting to change it you need to replace it at the below location in CB 1.x. With CB 2.0 you'll be able to select a new image from a dropdown once supplied to the correct location.

components/com_comprofiler/plugin/templates/default/images/avatar/

Ideally, I would like a default avatar for each membership plan is CB Subs.

One way you could do that is use CBSubs Plan Images plugin to assign an image per subscription plan. Next you'll need a delimiter field to check if they have uploaded an avatar using the IF substitution. If they have then display the avatar field. If they haven't then display the plans image field. Example of this as follows.


Further substitution usage information can be found in the below tutorial.

www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb

Re default,I can see a silhouette, but nothing is shown using different modules (I am displaying latest members).

Where? Please check the users profile and ensure it's displaying there. We don't have a module that displays just avatars in CB 1.x (CB 2.0 does). That's only available with 3rd party modules and those from S4J, but I've no way of guaranteeing their functionality.