I am able to export a Textbox rectangle as PNG or JPEG from figma screens Web App. However, couldn't find any way to export particular element as PNG/JPEG using figma APIs.
Figma API documentation doesn't help https://www.figma.com/developers/api#get-image-fills-endpoint
Found the solution.
Any element can be exported as png or svg or pdf using API given below
This api returns json response which contains the URL to download the image from figma s3 bucket