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It's not difficult. When composing a message, put the cursor where you want the photo to appear, then click on the little "mountain range" icon in the toolbar, which I've circled in red below.
This will open up an upload window. Navigate to the location of the photo on your computer, click Upload, and there you are. If they're all in the same directory, you can select multiple photos at one time and have them uploaded all at once. Before the upload actually occurs, you are presented with a screen that allows you to rearrange the order of the photos.
Keep your photos to less than about 500 kB in size. Many cameras take photos of 2 to 3 MB in size, which is way too large for uploading to a website. There are programs available (I use "Image Resizer for Windows") that can resize images for you.