How can i change the image in the game called "Memory Game"?

Hello everyone! I’m pretty new in unreal engine. I downloaded the project called “Memory Game” and i want to play this and be my training ground,. I want to change the image to my pictures just for fun. I already done it once but i couldn’t figured out how i did it. Everytime i import my png files and edit it to texture group, it seems that it lacks the material uasset, Can someone please help me how to edit the pictures? Your kind assistance is greatly appreciated. Thank you.