Please don;t cross-post. Your message will be read anyway. During the weekend it just takes a little longer before people read your post. Cross-posting is a waste of bandwidth, resources, time and makes this forum harder to use.
That said, there are a couple of problems with your approach.
First of all, you didn't post any code so it's hard to say why your height and width properties don't work.
Secondly, are you aware that the new dimensions of the image will cause the image to be distorted? 1024x768 has a different height/width proportion than 170 x 200, causing the image to appear stretched.
Thirdly, why do you want the browser to resize the image? The full image still needs to be downloaded, causing a waste of over 95 percent.
IMO, it's much better to resize the image at the server, and then send the scaled down version of it to the browser. You can use any graphics program to do this. Alternatively, with the classes in the System.Drawing namespace you can do this automatically as well.
That also answers your final question: don't put the large image on the web. Whatever you put on-line can be downloaded. In fact, once your users see the image in their browser, the image has already been downloaded and saved in the browser's cache.
By offering only the small image, you can kill two birds with one stone: your image displays correctly, you save a lot of bandwidth and you prevent your users from saving the large image (actually, that is three birds ;) )
If you post some code, we can try to find out why the height and width attributes don't work.
Everyone is unique, except for me.