i want lightbox that shows smaller image not depending on link

hello
i tried lightboxes but they all open largest size image, that is not good for web. that is when wordpress gallery was used. i tried to make one image separately, in case i can make it that way, i could replace all gallery with individual image codes. but in that way also image link is used, i can set there link to medium sized image, but it is not good, because way to original size image disappears, also in noscript mode. so, i want lightbox that shows smaller image not depending on link. it should be possible, because links to medium and other size images are standard, have prefixes like “-800×600”.