Here is what I was able to figure out...
Create an Attribute Expression:
var checkvalue = $feature.SOMEATTRIBUTE
var img_blank = 'https://blankimage.png'
var img_show = 'https://goodimage.png'
iif(checkvalue=='Yes',img_show,img_blank)
Then under CONFIGURE for Custom Popup, enter in any text you want. Add an image, and for the source, use the expression you just created.
Some text and here is the pic: <img border="0" src="{expression/expr0}" width="25" />
It works really nice for me, however it would be nice to have a little more functionality to better it.