Which one is correct:
When you see the image, scan it with the app.
When you see the image scan it with the app.
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"When you see the image" is an adverbial clause. "Scan it with the app," the main clause in the example ["When you see the image, scan it with the app."] is a complete thought. The adverbial clause attaches a temporal condition to the verb, "scan." When you put an adverbial clause before a main clause, a comma is required.
https://www.seattlecentral.edu/faculty/dloos/Grammar/adverbclauses/adverb_clauses_introduction.htm SEE: Using Adverb Clauses to Show Time Relationships.
Application of comma: When you see the image, scan it with the app. This refers to the image in specific. However, if you omit "," the meaning can take other ways too. Further, the main reason of "," is that it allows the reader to have a small pause so that the upcoming sentence can be related. In other words, we use "," when we wish to relate two different relatable objects which are complete individually( when you see the image ) + (scan it with the app).
Hope that helps