We recommend uploading vector artwork or high resolution images (300 dpi or better). Here is a video showing how to save 300 dpi or better images in Adobe Photoshop or Adobe Illustrator. There are similar ways to do this with other programs like Inkscape, etc.
You will have to find a higher resolution image on your own. Unless your file is an EPS file. Then all you have to do is convert it to an AI file.