Assuming your images were originally designed to work on a 800x600 resolution and you are keeping the same aspect ratio .. e.g. 4:3 then something like this ..
Scale# = GraphicsWidth() / 800.0
then
scaleimage Image, Scale , Scale
or
resiseimage Image, ImageWidth(image) * Scale , ImageHeight(image) * Scale
Scale# = GraphicsWidth() / 800.0
then
scaleimage Image, Scale , Scale
or
resiseimage Image, ImageWidth(image) * Scale , ImageHeight(image) * Scale