Hardware accelerated graphics cannot be stored in standard RAM, only VRAM. Construct already stores all textures PNG-compressed in standard RAM, and loads them in to VRAM when needed. The default is to load everything in to VRAM on startup, but you can set the texture loading to go layout-by-layout, so a large animation is only loaded in to VRAM when on that layout.
Loading animations frame-by-frame is not really plausible - the time it takes to decompress a PNG from system memory then upload to the video card, done for every frame, would create a noticable delay. It'd probably totally kill the framerate; this is why Construct loads entire layouts in to VRAM as a minimum.