Do you mean you use "px" values for the "width" and "height" attributes in the HTML "img" tag? E.g.:
Code:
<img src="my_picture.jpg" alt="my picture" height="200px" width="100px" />
If that's what you're using, it's not an EPUB issue - it's an HTML validity issue. The "height" and "width" attributes in an "img" tag should have simple numerical values - either the number of pixels, or a percentage value. So instead of:
you should just have: