There have been no changes in the way image import works in yEd. The information in How can I use my own node symbols in yEd? is still correct for the current version of yEd (which is 3.19 at the time of this writing).
Which version of yEd are you using (see "Help" -> "About")?
On which version of Java are you running yEd (see "Help" -> "About")?
On which version of macOS exactly are you running yEd (there are currently at least two "latest" versions)?
When you used image import successfully the last time, was your macOS already on the version it is now?
Anyways, you can try to reset yEd to its factory defaults, it does sometimes help with problems like this. To do that, delete your yEd profile directory (see "Help" -> "About" for the project directory location).
A word of warning, though: Deleting the profile directory will also delete all custom palette sections and all properties mapper configurations. If you do have either of these you do not want to lose, move the profile directory to a new location instead of deleting it. This way you can restore your old stuff in case deleting did not help.
