Here are some examples of how it's used:
* "The politician painted a rosy picture of the future, but many people were skeptical." (The politician was presenting a very positive view of the future, but people were doubtful about it.)
* "She had a rosy picture of her new job, but the reality was much harder." (She thought her new job would be great, but it turned out to be much more difficult than she expected.)
Essentially, a rosy picture is a positive but potentially unrealistic or naive perspective.