Are you meaning to resize the image to a percentage of it's current
size or as a percentage of it's parent html black (like it would act
if you just put percentages in the width and height attributes of the
html img tag)?
If you mean the latter, it's just a matter of changing the way
'image_tag' works so that you can feed it percentage as well as pixel
values but if you mean the former, I'm not sure if you can do it
without resorting to using rmagick (or similar) to find the width and
height of the image and then halve them and use that in the
'image_tag' size option.