you can take a look at :http://webaim.org/techniques/acrobat/converting
and http://webaim.org/techniques/powerpoint/#create
you will find what you need... But If the image does not have alternative text, it will typically be skipped by a screen reader. the accesibility check will give errors to all images with no alternative text, but not all are useful to screen readers, some of them you should leave them as is...