First, you need to re-duce your photo. There are many tools online to reduce a photo without losing quality. Your picture must be less than 256K.
Second, create your post and scroll down to attachments. You can attach up to three photos.
Third, after you upload your photos, you will see a button next to each one that says, "Place inline". When you get to the place in your post where you want your photo to be, simply click on that button.