Hi Maciej,
I'm surprised that the grid does this. It's very odd that it stretches the image bigger and even more odd that it does so ignoring the aspect ratio. You might want to Submit an incident to Infragistics Developer Support so they can check this out. It might be a bug.
The only way to alter this behavior is by using a CreationFilter to set the Scale property on the ImageUIElement that is displaying the image.
I too have this problem now.
When I used Infragistics 2008 Vol 1 the grid did NOT scale the images at all. Then I upgraded to Infragistics 2008 Vol 3 and now I have the same problem as the thread creator (images get scaled depending och row-height).
My question: is the grid supposed to behave like this or is this a bug?