Export Range of Cells in a Worksheet to Image
To take an image of a range, set the print area to the desired range and then set all margins to 0. Also set ImageOrPrintOptions.setOnePagePerSheet() to true.
The following code takes an image of the range E8:H10. Below is a screenshot of the source workbook used in the code. You can try the code with any workbook.
Executing the code creates an image of the range E8:H10 only.
You may also find the article Converting Worksheet to Different Image Formats helpful.