Your images are bigger to start with dear - to resize for the web you make them smaller! Even an 800x500 image has only 400,000 pixies at play - and your Pentax takes more than that
So in Image Size, set the output to 72, then the longest side to 800 (oops forgot the TP dimensions, someone will be along in a sec to confirm for you)
Save as a jpeg, and just reduce the quality until it's under 160k (this site's setting I think)