In other words, does Slicer 3D dismantle those images and insert them pixel by pixel with 8 bit channels (24 bits total) irrelevant of what format was used? Or does Slicer somehow preserve the format and thus Jpg sequence imported would require less ram?
No, png vs jpg compression only matters for storage to disk. If the images are the same resolution they will take the same amount of memory once uncompressed.