There are various ways to track down the image file number for an image added to any content section, any text editing screen, of your website. 

Here is one easy way.

go to your live website

• access the image on the page of your site
• right click on the image
  in Chrome, select Open Image in New Tab
  in Firefox, select View Image
  in Safari, select Open Image in New Tab
• a new page will open
• the url of the new page will look like this:
  https://fasoimages-4cde.kxcdn.com/13459_2386834l.jpg?cv=20170930hgts
• the 2nd number in the url, 2386834, is the image number, the file number
  NOTE:
  13459 is your account number
  l stands for Large (550 px)

go to your control panel

• click Artwork icon
• click on any image to open the image editing screen
• the url will look like this:
  https://data.fineartstudioonline.com/cfgeditworks_v2.asp?action=edit&KEY=2386837
  or this:
  https://data.fineartstudioonline.com/cfgeditworks_v2.asp?action=edit&KEY=2386837&START=&ORDER=
• replace the image number in the url, 2386837 with the number from above, changing the url to
  https://data.fineartstudioonline.com/cfgeditworks_v2.asp?action=edit&KEY=2386834
• Hit Enter/Return on your keyboard
• the image 2386834 will now show on your screen

also see this FAQ:
Add Image to Text Editing Screen