I'm doing self-teaching of brand identity design. While I look at wonderful examples on Behance, I found professional designers often use photos to further illustrate the brand identity. while photos over there are neat and often marked with the brand's logo, etc., I'm wondering how is this achieved, and where can I find photos like these? How do you put logos on business cards and legal size paper, etc.? 

An example: <https://www.behance.net/gallery/23271921/WALDEN>

As you can see, there are business cards, tote bags, candles... The last picture includes more items. How is this achieved?

Thx a lot!