When we call up a screen for the first time the image is not displayed; it appears as soon as we use the vertical scrollbar or press enter. Why is this so?

This is most likely a side effect of a bug in SAP GUI 7.20 that has been corrected by SAP with PL7 of SAP GUI 7.20. The vertical scrollbar was positioned at the bottom of the screen but the screen elements were displayed at the top, causing a faulty display of the GuiXT image.