Sitecore 7.5 brings some changes to the way images are resized on the server, and it may mean that you’ll find some images on your site aren’t being resized after upgrading. These changes are intentional, and provide better security for media requests, but depending on your past implementations…you may see this issue on your site.
Sitecore has always been able to resize images on the server by passing parameters to the image rendering controls, <sc:image> and <sc:FieldRenderer>.
In the past, the resulting image URL of this was the normal image path with a query string appended showing the parameters.
i.e http://mywebsite/~/media/System/Simulator Backgrounds/Android Phone.ashx?w=50
Shinks down to this….
From Sitecore 7.5 onwards, if you attempt to load an image URL directly (by just appending a query string like the one above), the image will not be resized, and the full image will be shown.