There are 2 methods of files / pictures attaching to Teams. One using the Attach icon, which as Hoard said goes to the files tab respective of each location. The other and most common used one, is a direct paste of images into Chat. You cannot look at these anywhere other than scrolling through chat, as they are stored in the Azure service via a chat entry just like the Text that is accompanied with them.
I took the literal definition of attach - not copy and paste :D :D :D
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