Sorry if my answer sounds too simple. But if you are not familiar with Photoshop might be happening to you that for whatever it is not loading your image at 1:1, that is, zoom 100%. Double click on the loupe icon to see it 100%, and then see if the chunky look dissapears (if so, file is ok, is just you were seing it slightly zoomed)
Other reason could be you'd be converting it somehow to indexed colors. Or it would be a transparent gif, but when seing in picassa or Windows is seen over white background, then on photoshop with the transparent checkerboard, small size, etc, it might look like chunky . In that case, just convert image mode to rgb, add a layer bellow, fill it with white color, and double click loupe icon to ensure a 100% zoome viewing.
It could be the way how you are saving it from picassa. Maybe not the final high res, but it's tricking you to save a thumbnail instead of what shows in the browser...