Pixelation (some may call it "jagged", "bitmappy" and sometimes, but incorrectly, "grainy") is most commonly caused by resolution inconsistencies or by re-sizing.
Basically, if you have 100 pixels in 1 square inch of space, and you change the dimensions to 10 square inches, you still only have the 100 pixels to start with. The nature of pixels is that they'll increase in size (not in numbers) to fit the new dimensions. Imagine taking graph paper, and blowing it up on a photocopy machine.
You can "upsample" or "downsample", and if you start with a high-resolution image, it will usually work, depending on the scale difference. Up/Downsampling involves increasing/decreasing the number of pixels per inch (or whatever measurement you're using) and inversely decreasing/increasing the physical dimensions of the image. In plain English, if you have a 2 x 2 image at 300 dpi, you can increase the physical dimensions to 4 x 4 if you decrease the resolution to 150 dpi. Basically, you've maintained the number of pixels. This won't always work, however, so be cautious.
Now, if your original image is 72dpi (common for a web snag), and you want to put it into a printed document (ideal resolution for print images is 266-300dpi), it'll look jagged, or pixelated if you keep the same dimensions. Upsampling from 72dpi is almost always a bad idea.
One "strength" of Word is that it can rescale images pretty well, but when you go into most other document editors made primarily for print (Quark, InDesign, etc.), resolution becomes a major issue (as dref explained - and dref, your English is just fine
). You need to keep source/destination resolutions the same (or very close). Given the choice, you want to go from a higher resolution snag to a lower resolution paste, rather than the inverse.
To be able to resize images and not worry about pixelation, you need to work with "true vectors" (Illustrator or Corel Draw or Freehand(??)). Photographs (scans and camera downloads) are raster images and work on a "pixel map"- kinda like graph paper.
Hopefully, I didn't confuse things
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