In this case I used Google slides, since I was creating the images there anyway.

In general, I often use simple image editors like Preview (macOS) or KolourPaint (Linux) to add prompt boxes and remove labels from images. It's very easy to copy-paste images into an editor, tweak them, and then copy-paste them into Anki.

Personally I find Anki plugins for images (ex. Image Occlusion) to be clunky overkill. All I need are colored boxes/circles and the ability to delete image text that might give away the answer.