To avoid the tile effect when importing images as a texture to Design, it is important to make sure the image used is formatted as a seamless image. Seamless images can be found on the internet, and you can create your own seamless image with a photo editor software (ex. Photoshop)
How to create a seamless image
The following link will redirect you to another website with a tutorial on how to create a seamless image using GIMP:
Tutorial: How To Create A Seamless Texture in GIMP - Game Art Guppy