How to Edit Image Size in HTML on Tumblr. ... When you need to give images exact height and width values, ... and an HTML editor displays the post's HTML code.

Fill in either the width or the height for your re-sized images; the other will change automatically, to maintain the aspect ratio. Click "OK." Advertisement. Article continues below this ad.

What you're looking for is probably to set width to 100%, then set max-height and max-width to the actual size of your image if you want it to never scale above that size: Code: ...