In the schedule, you can add two types of activities: standard activities and custom made activities. The standard activities are put in by Virtuagym and are used by everyone, so they cannot be altered. If you want a certain activity to have a picture of your gym/session/logo you can choose to create a custom schedule activity. Learn how to do so here.
When you add the activity, you can choose what picture you want displayed on the schedule. If you ever feel the need to change it, you can edit the activity by going to the business settings > schedule settings > custom schedule events and click on the activity you want to change.
Just remember, you cannot edit a standard activity, so if you want to use your own picture, you will always need to create a custom activity.