in a word, no.
what you save is all you have to work with. you are downloading a fixed amount of data and you can't do anything to increase the amount of
data.
with good image software like photoshop you can resample images to a larger size. there are free options (i use irfanview, see
http://www.irfanview.com) to do the same thing. but resampling will inevitably result in loss of image quality. all you can do is try to minimize that loss.
p.s. you signed up for a pay site and it only gives you 800x600 pics? that would be quite a scam. are you sure there aren't better download options? no full zipped sets at higher resolutions?
p.p.s. moved thread to "surfing tips"