Setting Image resolution of the images created by Powerpoint conversion service using PictureRequest class

I am using PictureRequest class to get the images of the presentation as,

PictureRequest request = new PictureRequest(inStream, ".pptx", PictureFormat.Png, outStream);

IAsyncResult result = request.BeginConvert(SPServiceContext.GetContext(siteCollection), null, null);

request.EndConvert(result);

After getting the response from the PictureRequest if I check the image size (in this case png) it has the resolution of 720x540. I want bigger resolution images. I searched all over the sharepoint central administration but could not find any setting related to Powerpoint conversion service which may allow to set the desired image resolution.

Could you please anyone suggest where I can specify the required resolution?

Many thanks in advance!