How to create custom renditions of images uploaded in DAM and how to export those images through JSON?

[#1]

I need to create a custom workflow or add a custom process step in the existing DAM update workflow such that I can provide my own image dimensions for renditions. After these renditions are created I need to send these images through JSON. How to obtain these images in JSON and how to create custom renditions?

[#2]

Please check the blog by one of linked in user:

Image Rendition in AEM

Also I am not sure why you need JSON data, as you can find all the details in metadata under jcr:content for example:

/content/dam/we-retail/en/activities/biking/cycling_1.jpg/jcr:content/metadata

~ Prince

[#3]

I am actually using AEM for authoring and content only. All the content authored in the page will be sent as JSON data to a third party. This is why I need to have all image placed in the page as JSON with all its renditions.

[#4]

The standard way in AEM to expose data in the JCR to other 3rd party destinations is use of Content Services. From the docs: 

"AEM Content Services output to destinations other than AEM pages, of content below page level (assets, fragments), in formats including but not limited to HTML, specifically now also JSON."

Adobe Experience Manager Help | Getting Started with AEM Content Services