To add an image to an existing block, need to identify if the block is a standard content block or a custom content block:
Standard Content Block:
If the content block is a standard block then you can replace an image but you cannot add an image to the block. Your options are to:
The first option is to find another standard content block that would work for the content and number of images that you would like to add. Once you create the new content block you can simply delete the one you don't need anymore.
The second option is to create a custom content block. See the "Content Block Designer - Working with Custom Content Blocks" article for more information.
Custom Content Block:
If the content block is a custom content block, then you can add or move elements in the block. See the "Content Block Designer - Working with Custom Content Blocks" article for more information.