I have created an application form allowing a person to insert a JPG image of themselves.
It works on Acrobat Pro DC but on Adobe Reader, the only option is for people to insert a PDF, not a JPG.
May be that the people uses a old version of Reader.
This used to be a big issue with Reader and whether or not it worked changed from version to version. But I'm pretty sure that from Acrobat XI onward the image (button) field will work for importing standard image formats into Reader. I know it works in Reader DC.