To target specific images, this gets a bit more tricky, but you can do that with a few ways. If it’s an image you are manually adding to a widget, post, or page, you can do an inline style on the tag like:
<img class="alignleft" style="border:none;"
….etc. Make sure “style” is after the “class”.
However, to show you where the border is being applied to, you can also choose which one to do the border:none with: