Image Conversion Settings - How do I specify pixels at the units?

I am trying to stop RoboHelp 10 mucking up (shrinking) my PNG screenshots when importing a FrameMaker 11 book.

The help file is at http://help.adobe.com/en_US/TechComSuite/3.0/Using/WS1b49059a33f77726-31aee1b1342288940d-7fd0.html

In the Image Conversion Settings dialog, there is a field for 'Maximum Dimensions' but the default units it wants appears to be points?

I want to specify it in pixels, as that is what my PNGs are composed of! How do I do that?

I'm getting the error message "Give a valid value for 'Width' field in 'Maximum Dimensions'. Valid values include measurements in pt, pc, cm, mm, in"

This is at odds with the Help docs, which includes the line "For example, if you specify the window size to be 800 x 600 pixels, you can specify the maximum dimensions to be 640 x 480, so that the images do not exceed the window size."