Which one is correct:
When you see the image, scan it with the app.
When you see the image scan it with the app.
“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.