Let's say you'd like to flag the current item in a list with a special icon, here's how you can do it.
First, when the user taps on item in the list that's associate with the data source, set the variable "mychoice" to hold a unique value from the current record. Let's say that's stored in [[ds:/mydb/items/uniqueid]] .
Secondly, in one of the image fields in the list (right image, disclosure icon or left image) use an expression like this:
In this case, each item in the list will display either "matches.png" or "does-not-match.png". If you'd prefer to only show an image when the two things match, use an expression like this one.
By omitting second value, you'll get an empty string back if the value in the uniqueid field doesn't match the variable "mychoice".