You don't quite understand. Construct immediately converts any image you import to the image editor in to PNG format, using its own PNG encoder. If you import a PNG file it is decompressed then recompressed using Construct's PNG encoder. If you import a PNG file that has been run through PNGCrush it is decompressed and encoded as a (probably larger) PNG file. If you import a DDS file, it is decompressed and converted to PNG etc. etc. etc. By the time you click run, everything has been converted to a PNG file and the runtime decompresses that on startup, so currently, the image format you use makes no difference at all to VRAM usage or how fast it runs, except that if you use a lossy format like DDS, you lose image quality - no gains, only loss.
When we come to make Construct 2, though, we will use external project images and the format won't be changed. So you can use DDS, jpeg, whatever is supported. That's a long way off though.
Also, frankly I think the DDS compression looks crap! Why would anyone want that?