How to resize thumbnail Natural Size?

I have a wallpaper site based on WP and having a problem in my search template, which is-Wallpaper thumbnail size that is being showed to users is ok, but that is taking to long to open because their natural size is more than enough, like if i have a wallpaper of resolution 1080 x 768-then the full size is being downloaded for users in small size.

Well, this is really eating my bandwidth and a big problem for my users.

I already tried to correct the thumb array with (300. 220) but that is also not working, is there anyone who can help me ??