Each peice of the audio/video puzzle is refered to as “an element.” Your text saved as a PNG file is just one the video elements of your finished project. For example, here in Hollyweird when you walk into a big post-production facility with a box of tapes, CDs, etc. that comprise your show, you would normally tell your online editor that all of the “elements” for your particular project are in that box.
The pixel dimensions of a finished DVD are 720 x 480, but if you create text elements at that size in Photoshop it does so with square pixels, the VGA standard. That will be resized because video uses non-square pixels. So, in Photoshop create files using one of the following frame sizes:
— 720×534, for DV NTSC
— 720×540, for D1 NTSC
— 768×576, for DV or D1 PAL
Good luck…
DRW