Sure! The two main things that affect the file size are the dimensions and the quality of the picture (if you have a nice camera, then chances are greater that it's a higher quality photo).
For frame of reference, the last image you posted was 2,592px wide, and the size was about 1.6 MB. So as a test, I ran your picture through the website I posted (using the 'normal' setting, and leaving the dimensions at the site's default 1200 x 900)... the file size went down to 75 KB!
I'd say that any image size under a 100-200 KB is just fine!