In Sitecore 8.1 update 1 there is a fix : Sitecore 8.1 Update 1 (An image’s URL became broken after changing its size in Rich Text Editor field. This has been fixed. (451366)) . Is there a place from where I can download Sitecore.Support.451366 patch?
Dora
Fixes are found on the Sitecore kb site
Unfortunately there is not an individual patch for this particular issue. This was a bug that was found in the 8.1 version and fixed in 8.1 Update 1.
You can either upgrade to 8.1 Update 1 or contact sitecore support and ask that they create an individual patch for this fix. (You'll need a Sitecore account to log in to this site)