You can't make them hold the image in their head for you to pick it out, though.

You'd also need to make them keep their head very still.

There are substances that block voluntary muscle movements. Pancuronium bromide, for example.

Don't think of the pink elephant.

I can't imagine pictures in my head, so regardless of how long I will think about pink elephant, you will see nothing.

For people like you, they can always fall back to the old "$5 wrench" method...

IMHO, thermorectal cryptoanalysis with soldering iron is much more effective for similar price.

OK. I'll just think about something else. I don't know why people think tricks like this work on anyone but the extremely simple-minded.

I don't know how your brain works, but for most people just hearing/reading the words "pink elephant" will cause their System 1 to immediately bring that concept into their consciousness.

This isn't "simple minded" it's just how brains work, and the alternative of having to actively think about a word to remember what it means, would be much slower.