The tiny photos you see in posts are often what are called 'thumbnails'. Thumbnails may often be clicked and they will open to full size.

This is not always true but usually.

If you upload a photo from your home harddrive you may see the full size image in your post and then a thumbnail of the same image at the bottom of your post.

If you upload a photo from an online photo storage site you will get the full sized image. This may be a problem with some of the newer megapixel cameras. Very large photos will cause the right edge of the forum to slide of the right edge of your screen.