No. You go get a design from AI or you create one with clipart. Style it in a design program with a font and phrase like goldfish image and the phrase "I have the memory of a gold fish." then you save that as an image. Not on a tee or anything. You upload to merch store, and they display it from the upload design on the merch they have in store. TBH, even just pure text designs with no clipart or image is popular if you say the right thing.
Create image in photo/image editing program. Upload it per size and resolution requirements of marketplace sites. You can promote your designs once they go live on socials too then. Its like having a free store with all your merch in it, with people more oganized than you taking care of all order backend porcessing.
With cafepress and bonfire(new one i came across recently) you get multiple products so its not just tees you can promote on socials, you can do posters, clocks, mugs, and other things from your stores