Yes, images have to be at least 80 PPI (pixels per inch) and a minimum size of 640px x 640px. Of course, the higher the resolution of the image the better the print result.
You can't upload TIF or PDF files. Either save those files as JPG or PNG and upload them, or contact us if you want to print from the original file types.
If your image is lo-res or if you can’t upload an image, let us know so we can help.