Pixelation is not caused by your software. It's caused by the actual characteristics of the picture file.
There are two criteria that decide the quality of a picture - its physical size and its resolution (which is usually measured in dots per inch).
To increase the quality of the picture, you need to increase the number of dots per inch. But you can only do that really successfully if you reduce its physical size. In effect, what you're doing is squashing the dots closer together.
Depending on how they're set up, digital cameras will sometimes shoot a picture of very large physical size - say 50cm x 30cm - but at low resolution - perhaps 72dpi. If you use image editing software (I use Photoshop) to increase the resolution to perhaps 300 dpi, you'll find that the picture is physically a lot smaller, but has much better resolution.
Some software can interpolate a higher resolution for a picture, but it is actually "guessing" what the infill dots should be, and this is rarely very satisfactory.
In the professional field there must be some pretty remarkable software. Recently I needed to print an A4 image urgently and would you believe it my trusty R300 played up. In desperation I e mailed the image to Photobox. In so doing I forgot to switch of Shrink Pic the automatic re sizer-So the must have had a scrappy image from me. So I did not expect much in return. But to my amazement I had 3 superb A4's Not a pixel to be seen.