I don't even see a Wall at your site.

So what I think you were referring to was the Newsfeed, in that case you would use the codes as follows:

.ow_newsfeed_large_image .ow_newsfeed_item_picture {
max-width: 460px;
width: 100%}

still showing smaller pic

Try:

.ow_newsfeed_large_image .ow_newsfeed_item_picture {
max-width: 460px !important;
width: 100%!important}

thanks, it works :)