The menu items are created in unit mainbase in the ide folder of your Lazarus installation, the "Source" menu in TMainIDEBase.SetupSourceMenu:
procedure TMainIDEBase.SetupSourceMenu;
var
ParentMI, SubParentMI: TIDEMenuSection;
begin
with MainIDEBar do begin
CreateMenuSeparatorSection(mnuSource,itmSourceBlockActions,'itmSourceBlockActions');
ParentMI:=itmSourceBlockActions;
CreateMenuItem(ParentMI,itmSourceCommentBlock,'itmSourceCommentBlock',lisMenuCommentSelection, 'menu_comment');
...
CreateMenuItem(ParentMI,itmSourceCompleteCodeInteractive,'itmSourceCompleteCodeInteractive',lisMenuCompleteCodeInteractive);
...
The first CreateMenuItem call has as last parameter the name of the icon, here: "menu_comment". The second does not. Therefore, it gets the default icon.
All you have to do is to go to folder images/menu, find the icon that you would like to have for the "Complete code" item, add its name (without ".png") to the corresponding CreateMenuItem call, and rebuild the IDE.