Actually I even didn't succeed in displaying the pictures in the size in mm I entered. I already read in another post, that sometimes the display information is not read correct?
Computer displays work in terms of pixels. Pixels have no strict relation to any real, physical measurement unit (mm, cm, inch, etc.). I.e., what a pixel corresponds to in terms of mm, etc. depends on factors such as the physical dimensions of the display (visible, *not* nominal, diagonal), its set resulotion, its pixel density, dot pitch, etc. In other words: It will be different for every display. Except for screen resolution, none of these are known to any software.
The OS functions converting pixels to physical units are based on very general assumptions and results may vary considerably in accuracy due to the above.
What to do about this?
Nothing really, short of including some sort of calibration routine in your script (cf. the Spatial Delayed Response Task available from the Task Library). However, (1) you'd need Inquisit 3 to do that and (2) it would be pretty unsuitable for web use.