We finished updating to Omeka S 3.0 and got all of our themes to the latest versions when we noticed that images are not being displayed as they used to. After looking around we noticed two potentially separate issues.
Items displayed in an item showcase block in any of the 4 built in Omeka S themes are not resized properly if you add more than one item to the block (medium and large size are the same size as a square, just not squared). They seem to be sized correctly (albeit displayed vertically) in a new theme we got called Foundation.
This isn’t going to be a matter of settings, but rather the styles in the theme. The actual medium, square, etc. images are the same sizes they were before, but the theme is limiting the size it displays them at. It’s possible your actual default sizes for new images have changed, but that wouldn’t be something the upgrade itself changed: those sizes are specified in the config/local.config.php file.
Editing the theme’s CSS or overriding it with the CSS Editor module are options for changing how the themes display the images.