white boxes instead of graphics on some webistes

Hi. I use a 3G dongle on 49€/month contract (movistar.es). The past week or so, I have noticed that on some websites the thumbnails and small graphics are coming up as blank white boxes. I have read that some ISPs compress images on certain webistes (Ancestry.com for one!). Is Movistar 3G doing this to save bandwidth? Is there a way I can turn this off? Change apn for example? Regards Jerry