Why 6MP in Pentax istD is not good enough?

Recently I tried to upload some images to a stock photo site. The site is very picky in the technical details (I am glad that they are so), and they require the uncompressed file size to be no less than 48MB, that means the long side should be >5000 pixels (assume a 3:2 aspect ratio).

I enlarge some images from istD + DA16-45 @f/8 accordingly. The resultant image is too soft and been rejected by the stock photo site. See the 100% crop below (click image to enlarge):



I admit the image was shot with jpeg mode and not best from istD, but the RAW file don't give me much room either.

Then I try another image from K20D + FA/77@f/4 (also shot in jpeg mode, and also need a little bit enlarging in PS to meet the 48MB requirement). I am still awaiting the response from the site, but the 100% is much sharper (click image to enlarge):



Cross my finger....

p.s. I will give the istD one more chance and find some RAW file and do the same up-sizing... but I don't think it will look good....