help needed - high resolution pictures and filesize problem

[turf]

i have some high resolution pictures that i want to make available for download to members of our sites but when i try and zip them the size of the zipfile doesnt really change much compared to how much space they take up unpacked..

So my question is is there a way to get a high resolution picture to shrink in filesize without loosing quality ? i have tried to strip them of exif info but that doesnt really make any difference as each file only shrink by aprox 200kb a piece..

To do a size comparison i checked out the size of 73 high resolution from femjoy and they state an aprox 75MB download for the images where as my zipfile would take up 260MB+ - so either i am doing something wrong or my pictures have too much junk in the trunk that my stripperprogram doesnt remove ?

any info on what to look for or how to shrink a hi resolution picture in filesize without loosing quality ?