In Microsoft Teams Free, there isn't a dedicated "gallery" feature like the one you might have used in Skype. Instead, you can access pictures shared in your chats or channels by scrolling through the chat history. If someone shared images in a conversation, they should be visible in the chat thread.
Currently, Teams Free does not support a specific "photo tab" to view all pictures in one place. You may need to manually search through your conversations to find the images you are looking for. If you have saved any images to your device, you can also upload them again to the chat if needed.