This could be irritating for some editors as the icons for the Sitecore Internal Link dialog and Hyperlink Manager are the same.
You can change it by changing the Icon field of the "Insert Sitecore Link" item: /sitecore/system/Settings/Html Editor Profiles/<name of the profile that you use>/Toolbar 1/Insert Sitecore Link
Here is the image you can use as a source.
Copy this image to \sitecore\shell\RadControls\Editor\Skins\Monochrome\Buttons\
Here is the result: