You will need
  • image editor, picture
To make preview pictures, you need to upload a large image file in a graphics editor such as Adobe Photoshop or similar. Then reduce the size of pictures in pixels. Usually used for preview pictures resolution about 200x200 pixels, but for your purposes, perhaps a different size.
Save a smaller version of the picture in a separate file. In some editors you can select the compression ratio. If you send a picture to the Internet, you can apply a little more compression to the image load faster for those who view your text.
For preview do not have to reduce the whole image, some prefer to cut a small piece and as a preview to use it. It depends on your taste preferences and goals of the text.
Then upload both images to the file hosting. You need to get the direct address for both images. Code to embed the images:
<a href="direct link to big picture"><img src="direct link to small picture preview"></a>