gaelbarbe , From within your meeting, go into
- "More Actions"
- Apply background effects
- +Add new
- <choose the graphic you want to be your background>
- The graphic you chose will show up at the bottom of the available backgrounds
Anyone can create a background. Our marketing department created some company-branded backgrounds for everyone in our company to choose from.
It's basically any graphic that is 1920x1080 pixels. This means you can also search the web for any pictures with those dimensions.
I highly recommend that you take a look at the training & support resources that MS offers. They are really good and answer many questions like the one you asked. From within the Teams local app, in the bottom-left corner is "Help". From there, you can choose from various options.
You can also find a lot of great support and training materials here: https://support.microsoft.com/en-us/teams (you can go to your regional version of the website if you aren't in English / United States). For example, simply change the en-us in the URL to de-de and you're be at the local German / Germany site.