When resizing images you have 2 options:

1. Thumbnail types
2. The Legacy Thumbnailer

Each has its pros and cons and while I personally much prefer using Thumbnail Types, it really depends on the situation.

Thumbnail types

Thumbnail types are defined in your dashboard under dashboard/system/files/thumbnails.

Once thumbnail types are defined, any image you upload to the file manager automatically gets a new version created for each existing thumbnail type.

You can also decide to create versions for a thumbnail type based on whether an image belongs to a file set or not.

Pros

By the time you load your page with a gallery on it, all required versions on your images already exist and are readily available so there's no unnecessary process slowing down your page load.

If you modify a thumbnail type and regenerate your images, all galleries using that thumbnail type will automatically start using the new images with the new size.

Cons

The more Thumbnail Types you create the more images you'll have on your server. It's especially true for older versions of Concrete5 when the possibility to limit certain types to certain file sets didn't exist.  That's potentially a lot of images on your server that you might not need. So use responsibly.

Legacy Thumbnailer

That's the old way of resizing images and it's all done on-the-fly while the page loads and images are requested.

Pros

Images are only created when needed so you won't have a bunch of unneeded files on your server

Resized images are saved in the cache and you can select to not empty the image cache when emptying the cache so they won't need to be generated again and again

Cons

Because images are generated on-the-fly, the first time it can considerably slow down your page load, especially with 30 images or more.

If you want to change your thumbnail sizes you'll have to change them one by one manually

If your thumbnail cache is emptied every time the cache is emptied the images have to be generated again and again slowing down the page each time.

So, which do I choose?

If you only have a few small galleries and are concerned with the potentially big number of images generated by thumbnail types then use the Legacy Thumbnailer.

If you want to make sure you always have the best performance no matter what and are using a newer version of Concrete5 so you can limit thumbnail types to specific file sets than use them.