For images uploaded to Blogger itself, adding the query parameter ?dl=1 at the end of the image URL will cause them to be forcefully downloaded. Another way for doing this is, by suffixing -d after the sizing parameter in the image URL (aka s1600) For example -
<a href="https://1.bp.blogspot.com/-k1bwGQauV6I/WLyURMAuU9I/AAAAAAAAAD0/k_ZcX4sr_bENoB2P3P8MTEfN-ymfa3_ugCLcB/s1600/en_Contempo.jpg?dl=1">Download this Image.</a><a href="https://1.bp.blogspot.com/-k1bwGQauV6I/WLyURMAuU9I/AAAAAAAAAD0/k_ZcX4sr_bENoB2P3P8MTEfN-ymfa3_ugCLcB/s1600-d/en_Contempo.jpg">Download this Image.</a>