Silvio,
on init the first positions of the images are shown.
Next You move the images on a painted background of the images shown at startup.
Because the images are transparent, the not covered images BEHIND the new ones are shown.
It will only work, if You RESTORE the original brush of the window or dialog inside the used area
BEFORE painting new images.
Using Your sample, You can REFILL the used area with green, before painting the new images.
It can be a colored bitmap ( green ) with the size of the used area.
A other solution : a colored background of the images ( green )
It can be done easy with PIXELFORMER.
best regards
Uwe