The error "Making this change would require changing code that is locked by a template or translator" is a common error when using templates that does not really have a solid fix, see my post here for a workaround:
showpost.php?p=18157&postcount=2
to restrict validation to a certain file extension or file size, you will need to use the Server Validation included in CSS Form Builder.
Use File Extesnion validation to restrict to jpg or gif files, and number validation to restrict to specific file size.
When you click the lightning bolt next to the server variable, select the fileField bindings with the lightning bolt icon, see the attached screen shot for an example of the file size binding.
