Can I edit or make my own portraits?
No.

Can I make my own heroes?
The only heroes available are those provided in the editor. While you can rename them and modify their statistics, you cannot create a hero from scratch or change portraits or special abilities for a hero.

Will there be a patch that includes the ability to create new units?
Units have many different statistics and animation-specific scripts that need to be incorporated into StarCraft on a very basic level (i.e., in the core code.) Future Blizzard titles will provide for more flexibility in this area via the provided scripting language.

Will there ever be a unit sound editor like there was in War2?
Although we realize that this was a popular component of War2, time constraints prevented us from adding this to the initial StarCraft release. It is possible to replace the built-in game sounds with the assistance of the StarCraft Sound Utility. Check the Help file included within the zip file for instructions.

Why can't I modify other unit statistics, like firing range and movement speed?
In order to be effective, the AI makes certain assumptions about the relative strengths and performance of the various units. In Use Map Settings games, the AI weighs the value of different units based on their attack strengths, cost, hit points, and other factors. Certain factors, such as movement speed, are not so easily factored in, and to keep the AI competitive we had to leave certain abilities at specific levels.

How do I change the advisor portrait like you did at the end of the Protoss campaign?
Become a Blizzard programmer. ; )
(This was hard coded in the game and cannot be achieved by using StarEdit).

Why was the ability to add splash screens hard-coded instead of being part of the campaign editor?
The decision to add pre-briefing splash screens was made at a very late stage in the project, and we simply did not have time to design, program, and fully test a simple interface for choosing and customizing pre-mission splash screens.