Regardless the extension that you give to the file, SavePanelDisplayToFile produces a bitmap object and creates a bitmap file on disk, not a png one. You can check it looking into the code for the function in toolbox.c file: first of all the function obtains the bitmap object form the panel, next saves it to file. Now, it seems to me that bitmaps in CVI always follow the rule stated in on line help for PixelDepth field in NewBitmap function:
Regardless of the color depth that you specify, the new bitmap matches the color depth of the display screen.
Now, if you cannot reduce your screen color depth ancd indeed you want to obtain a smaller resolution you should run into the process or mapping 24bits into 8bits colors, or find a library that does this for you, next save the bitmap to file. It may not be the easiest thing of all...
Finally, you may find useful to use new SaveBitmapToXXXFile in the User Interface library, which permits you to create true PNG or JPEG files instead of BMP ones, but I'm afraid you will have the same limitations as of color depth since all these functions rely on bitmaps.
