What you upload to My Game Assets is there to be used. Here are the places it shows up.
#In a game
Some games let you bring your own media to an event. On the event's Photos & Assets tab, the Custom game assets card takes .jpg, .mp3 and .mp4 files. See custom media for an event. Which games use sponsor or advertisement media, and how, is explained in each game's own guide.
#In an email
The email editor has a Media Library link, so you can choose one of your uploaded images for a message. See sending an email.