Resizing all images

I have just changed the theme on my blog, and because of a small page width, it breaks the layout. Most of my earlier images (attached in posts) are around 600px in width, and now I need to resize all of them them proportionately to 450px. How do I resize them all at one shot? Is there a plugin for this?

How to extend Media Library (WP 4.4)

I am trying to implement a new tab next to Media Library. I couldn’t find solution to implement this. I found an example how to implement a new tab in this link: https://gist.github.com/Fab1en/4586865 However the tab doesn’t show up in the modal form triggered by Add Media button. But if I call the media popup for Featured Image or any other custom button the “New Tab” I created shows up. I am really lost here, not sure why those tabs show up in one place and don’t in another.

Increase Size Limit of Media Files WordPress 4.1

I have edited (created) the php.ini file and the .htaccess file in accordance with this post. I have also found a plug-in (called Upload Max File Size) that claims to raise the file size limit of media files. With the plug-in activated, I see on the add new media screen that the limit has been changed. However, when I attempt to upload a 2.7MB image, I receive the error: