Ytdyklly

I have a Canon Selphy printer and when trying to print images that have been edited on my computer a question mark appears in the viewer screen. It appears that the EXIF thumbnail in the image has been stripped out.

Is there an application to generate the EXIF thumbnail for an image?

I have tried using JHead but that only regenerates the thumbail if there is already one present.

You can use the -g option of exiftran:

exiftran -g -i image.jpg

The -i option does in-place modification to the file.

You can use nconvert:

nconvert -overwrite -buildexifthumb foo.jpg

The task can be completed with exiftool (for updating EXIF) and ImageMagick (for crating thumbnails).
First let's download and install/extract both programs. Put exiftool to any folder and include it to PATH (Imagemagick folder was included to PATH during instalation). Next run cmd change to a folder with images and run

for /r %a in (*.jpg) do (Magick convert "%a" -resize 256x256 thmb.jpg &  exiftool(-k).exe "-ThumbnailImage<=thmb.jpg" "%a")

or with % doubled to be used in .bat

for /r %%a in (*.jpg) do (Magick convert "%%a" -resize 256x256 thmb.jpg &  exiftool(-k).exe "-ThumbnailImage<=thmb.jpg" "%%a")

Explanation

convert creates 256px thumbnail thmb.jpg and exiftool writes it to original .jpg

Note

exiftool copies original file to .jpg_original which is supposed to be deleted after checking that the update was completed succesfully.

Try this simple tool called Exif Thumbnail Creator. It's a software to create or reconstruct EXIF thumbnails.