Tiles can typically accept images of non-matching sizes up to 1024x1024px, and they'll be auto-cropped. If an image is larger than that in any dimension, however, it won't appear.
Images will also be ignored if they're over 200KB in size. This can become an issue with PNGs at wide tile dimensions, so check that carefully. If you have a photographic image, you'll get a smaller file size using JPEG and suitable compression.
You can reencode images using the APIs in Windows.Graphics, but it shouldn't be necessary. First check your dimensions and file sizes. I would also suggest trying the images inside the
App tiles and badges sample, as that would eliminate any glitches in your own code that might cause a problem.
Kraig
Author,
Programming Windows 8 Apps with HTML, CSS, and JavaScript,
a free ebook from Microsoft Press
Also see
second edition preview