Branching into a new discussion,
Hi Thanh,
Thanks for reaching out to the Stock team. If you are using any unmodified images from our Stock website you must include a credit line to the photographer, however, if the images are modified, the credit line becomes optional.
Additionally, I would like to let you know that if you are showcasing your work to the larger audience in the form of your mobile app or if you are commercializing your mobile then you must include a credit to the photographer.
Please note: It entirely depends on what type of modifications are being made, for example, if it's just the colour change then the credit line must be included. However, if the changes are on a wider level, the credit to the photographer becomes optional.
Hope this information helps!
Feel free to update this thread in case of any additional questions.
Regards
Twarita