In the beginning every single sale or even every single acceptance is a moment of celebration. As your stock photography career evolves the pacing the milestones spread out - 50 acceptances (become exclusive or not?), $100 in sales (first payout), 100 online (sales become more regular), 500 sales (now we are talking) etc.
My latest milestone to crow about is 1,000 sales!
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Photo credits: Peanutroaster.