To add photos to your proposals, open up an existing proposal using the Edit Proposal button or click New Proposal button. Next to the line you want to add a photo to, click Add Photo. This will allow you to add a photo that you have saved on your computer.
Note that images should be less than 200kb. If you need to shrink down a photo you have on your computer to a manageable size, check on this tutorial on reducing the file size of your images.