First you need to understand the difference between the two and you would see that in general this question is meaningless. KMZ (compressed JREG) is a raster file (image) compared to .img which is a vector file compossed of points, lines and shapes. Read this:
What the software mapwell is doing, as Caty suggested, is a trick - vectorising the image. This means that by using an incredible amount of ponts, lines and shapes a simmilar vector file as the image is createn. But the bad thing about this method is that the created end vector file is incredibly large and heavy for rendering from the unit.
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If you can write what is the end task maybe we can suggest a smarter way.