I see people with custom images in their game and I want to know how to do it
You cannot import custom images, but you can use images from the Gimkit documentation. They’re called icons, you can see them here:
You can also use the new Item Image device, but those only allow you to put images from items used in Gimkit Creative, not custom ones.
Here are some cool ways to make your own images:
Dot Art In GKC for Custom Images
How to add custom images to your vending machines. (Difficulty: 
)
Unicode Symbols (for your Game)
[ 
] Every Single Image in Gimkit