Just to make sure - we're basing this on the episode itself, not necessarily its title, yes? (he asks hopefully ... ) Some titles are kind of specific to the crew involved (i.e. "Spock's Brain", hint, hint
, or "Sons of Mogh"), but could still be recognizable if used for another series.
To be honest, I did not think of those types of eps, and figured that you should be able to work with the existing title. So the existing title's must be used, but in the examples you cite, where a specific element of that crew is used, then it can be changed to better reflect your entry. So if you want to do "Janeway's Brain" or "Sons of Soong", it would be okay.
On another note, I see a lot of photoshopping with screen caps and whatnot and that's cool if that's how you want to go......however, feel free to be more creative. For example, Solarbaby is doing "The Changeling". If it were a book cover, she could easily use the DS9 logo, the ep. title, and a puddle of goo. So don't feel like you're limited to having to use screen caps to do this.
It could minimalist artwork, but as long as it makes you go "wow, I wonder how cool that would be?" then you've done the job.