If you select an image file and choose “Send To — Mail Recipient” in the context menu, the Send Pictures via E-mail dialog appears to ask you if you want to resize the image. To disable the dialog and to directly open the email client with the image attachment, follow the instructions below. This tip was hard to find with Google, so I pasted it here verbatim:

To disable the resize dialog for .JPG file type, use these steps [Reference]:

1. Click Start, Run. Type regedit.exe and press {ENTER}
2. Navigate to the following location: HKEY_CLASSES_ROOT\.JPG
3. Backup the key by exporting it to a file.
4. In the right-pane, double-click PerceivedType and assign a blank data to it.
5. Close Registry Editor.

The same procedure applies for other image types. The original source also gives this caution, however, I did not encounter the mentioned problem:

There may be some minor side effects of using this method, as the PerceivedType string is another important file class information. PerceivedType string helps Windows determine the actual Type (image, video, audio or whatever) for a file. In case you experience image preview issues or any other problems after following the method in this article, you can revert the setting by changing the PerceivedType to image. Alternately, type REGSVR32 SHIMGVW.DLL in Start, Run dialog to regain the functionality.