For those of you completely confused by the utter lack of documentation for this class (including notes being dropped, probably this one included), the differences between any "setXXX" and "setImageXXX" seems to be entirely dependently on HOW the image was loaded.
If the image was loaded at object constructor time, it seems that you use "setXXX" - if you loaded it after the fact (such as via "readImageBlob") then you use "setImageXXX".
This is irregular, however. For example:
$image = new Imagick();
Note that we use "setResolution" not "setImageResolution".