maniu

Very useful WordPress plugin to resize images before they are being uploaded. Just drag and drop big images and don’t worry about your bandwidth or server space usage.
Everything is configured in “Settings” > “Media”.
It is using power of modern browsers to do the job – Chrome and Firefox would use HTML5 method and other browsers – Flash. 
Generally it looks like Chrome is the best for the job (<a href="http://www.plupload.com/" title="plupload.com" rel="nofollow ugc">plupload.com</a>)

Browser Resize Images

dmry

This plugin enables you to automatically resize every user-posted image which is larger than given dimensions. Original plugin writen by <a href="http://www.ncode.nl/vbulletinplugins/" rel="nofollow ugc">Jorrit Schippers</a> for vBulletin.
Some features:
<ul>
<li>No resizing</li>
<li>Enlargement in the same document</li>
<li>Enlargement in the same window</li>
<li>Enlargement in a new window</li>
<li>Enlargement in modal window</li>
<li>Any JavaScript Framework is not required</li>
</ul>
It also adds an information bar (which is not displayed when the image is too small) giving information about the original dimensions and filesize.
Localization:
<ul>
<li>Belorussian (be_BY) – <a href="http://www.fatcow.com/" rel="nofollow ugc">FatCow</a></li>
<li>Arabic (ar_AR) – <a href="http://linuxawi.com/" rel="nofollow ugc">ZAH3R</a></li>
<li>Japanese (ja) – <a href="http://blog.osakana.net/" rel="nofollow ugc">osakanataro</a></li>
</ul>

nCode Image Resizer

This plugin enables you to automatically resize every user-posted image which is larger than given dimensions.

Browser Resize Images vs nCode Image Resizer