chr.schmitz wrote:1. Does every screen provided by a sample set occupy memory or only the screen(s), which are currently displayed?
All images specified in the organ definition are loaded whenever you load the sample set. (That's necessary for sufficient real-time performance.)
chr.schmitz wrote:2. Does it make sense to compress graphic files (E.g. http://compresspng.com. I tried different tools, but this was by far the most effective.) in order to decrease the memory footprint of the displayed image set?
As far as I know only the image size and bit-depth (8-bit colour, etc.) affects the memory footprint, since I believe the image is stored in memory uncompressed (again for real-time performance reasons). The underlying multi-platform library and operating system handle those things. File format and compression are only relevant for loading/saving images (disk space, loading speed, etc.).
I wouldn't really worry too much about it anyway, since image files are usually tiny compared to the sample data. I'd suggest just using PNG files, which are reasonably well compressed anyway (and thus may load slightly faster and take up less space in installation media), loss-less, and support transparency.