It's easy once you know the routine.
1. Make sure the photo file is less than 100K in size. There are more than a few programs and sites that will resize it for you, I use Photoshop.
2. Click on the upload a file button in the lower RH corner of your post.
3. Open the folder where the pic is, I always put it in a folder on my desktop for ease of finding it.
4. Click on the pic and either add it as thumbnail or full image.
5. Post
The only issue that will prevent it from uploading is the file size, must be less than 100K in size.