Update: I implemented the fix recommended by gsoules in this thread on the same issue, making the change in \application\libraries\Omeka\File\Derivative\Strategy\ExternalImageMagick.php to add the .exe extension to convert. This worked to get Omeka to recognize the ImageMagick directory path.
However, it didn’t work to get Omeka to actually display images. I tried substituting magick.exe for convert.exe but that didn’t work, either.
Finally, I implemented the recommendation from jflatnes to add this line to application/config/config.ini:
fileDerivatives.strategy = "Omeka_File_Derivative_Strategy_GD"
That worked to get images showing up. 
Since Omeka recognizes the directory path for ImageMagick, however, there’s perhaps some adjustment I can make so that it uses ImageMagick for conversions?