If the image is a picture any text on it will scale with the picture. If you want to be able to control the text size, font, color, and maybe even the actual text, you are best to not save it as a picture. The drawing canvas will keep anything that is inserted onto it as a group. If you select the box that outlines the canvas and copy it you can paste it into PowerPoint. Do a regular paste, not paste as picture. You will have access to edit the image, text, etc.

If you really need it as a picture, you can select paste as picture into powerpoint and then right click on the picture and do a save image as. It is a round about way to get the picture, but it is the only way I have found to do it.