Add the possibility to upload pictures that are used on cards

[LukasKM]

• [ ] The feature was discussed in discussions of this repository.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Description 💡

The pictures used on cards have mostly no relevance to the projects. It would be great if the users could modify the pictures on the cards and replace them with their own ones. It would facilitate the user acceptance of the system.