The “filter” is just a type of hook. A “hook” allows you to do something at a specific time or place in the execution of WordPress or a WordPress theme. In the WordPress theme coding world, there are two types of hooks: ACTIONS and FILTERS.
Actions are hooks that are executed at specific places within a theme, such as the header or footer.
Filters are hooks that are used to modify text or other attributes before adding the content to the database.
In this case, whenever you upload an image in WordPress, the image is duplicated and cropped according to the images sizes specified in the following places:
WordPress Settings > Media
Jump Start parent image sizes
Your Jump Start child’s image sizes
There are often many different ways to accomplish things, and in this case, we have four choices:
1 – Modify the WordPress Media Settings
2 – Add an image size using the WordPress function called add_image_size: http://codex.wordpress.org/Function_Reference/add_image_size
3 – Use the code snippet Jason has provided above. The filter he is referring to is the add_filter command (as opposed to add_action). You are “hooking” into the filter which Theme Blvd already uses to generate image sizes and adding some of your own! Cool!
4 – Use the Theme Blvd Image Sizes plugin, which will change the image size for default Theme Blvd images, such as the small thumbnail used in a Theme Blvd post list on the blog page and specified in the Appearance > Theme Options.
If you want to change the way your blog post featured image thumbnails or large images display in the post list or post grid, then use option #4 – the Theme Blvd Image Sizes plugin.
Note, however, that once you change the size using this plugin, any previously uploaded images will not be resized. This is because WordPress sizes images upon UPLOAD, not after the fact. It’s a function of the upload feature in WordPress that generates the various images sizes specified by WordPress and the theme.
To regenerate your thumbnails, you can reupload them, which might be a pain if you have a lot, or use the Regenerate Thumbnails plugin: http://wordpress.org/extend/plugins/regenerate-thumbnails/
However, if changing the size of slider images, if the original images were too small, regenerating the thumbnails will not make them larger, and you will need to upload appropriately sized images manually.
I hope this helps. It is a lot to get one’s brain around with lots of variables and different solutions for different applications.