William P. Davis

This plugin allows you to duplicate images you have in the media library and attach them to a new post, rather than having to create a new copy of the image and upload it to WordPress again.
It works by copying the post and its metadata. It does not actually copy the image. Any changes you make to the new copy of the attachment — updating the caption or cropping, for example, will only be applied to the new attachment, not to the original.
This is useful if you query posts for its attachments to display photos, or if you make use of galleries and have old images you might want to include, among other things.
To use, go to the Library tab in a post. You should see an ‘Attach’ link.

BDN Duplicate Images

Allows you to duplicate images in the media library and attach them to the current post.

Sean Butze

Many users do not compress or resize their images before uploading them into a post, and oftentimes WordPress’s maximum upload limit of 2MB-10MB is still too large to prevent the insertion of photos that can signficantly slow down a website.
WP Image Size Limit allows an administrator to set a custom file size limit that is specific to image files and smaller than WordPress’s general file size limit.
This is especially useful when you need to put tighter restriction on image uploads but want to preserve the ability to upload larger files of other formats (audio, video, etc.).

WP Image Size Limit

Just what I needed to tamp down those designers who think it's okay to use 20MB images!

Brilliant

Adds a new setting under Settings -> Media where an admin can set a maximum upload file size for image files.

BDN Duplicate Images vs WP Image Size Limit