Here is a 'quick-and-dirty' method for adding captions to images in your Blogger post.
The thing to remember is that by invoking 'Add Caption' to your image, you are placing an old-fashioned HTML table around the image. This will also allow you to resize to some preset sizes and align the image more easily with the text. You can see these extra options in the pop-up menu in Fig 2.
Fig. 1: Here is an image already uploaded. Can be from any source
Fig 2: Click on the image itself and a pop-up menu will appear. One of the options is 'Add Caption'
Fig 3: Some placement text will appear and just place the cursor and type over