I have a site where I list lots of thumbnails (photos) on the front page in a grid layout. Each image is 120px x 120px. What is the best way to keep them sharp on high resolution devices?
Should I make them 240px x 240px and then in the code resize them like:
<img src="file.jpg" width="120" hgith="120">
I guess jpg is better than png because of the files sizes?
yes you can use it like you mention. no matter you r using png or jpg.
you can also set thumbnail size from WordPress setting.
You can try following code
https://codex.wordpress.org/Function_Reference/get_the_post_thumbnail