You can export images from your design app of choice. Use this in your prototype to provide accurate and good quality user interface.
Transparency works and these objects are interactive like anything else.
How to upload a new image file
Under the 'add object' dropdown, select image and custom image.
Then upload your image. Once uploaded, your image can be selected again in the future, from a gallery in the image modal.
Now you can move and scale your image.