Lyn image browser supports Mailplane
Mirko Viviani just released Lyn 1.0.2 with Mailplane integration. Lyn is lightweight and fast image browser that natively supports all popular image formats like JPEG, TIFF, JPEG 2000, PNG, TGA, RAW, HDR, OpenEXR, PPM, animated GIF and any other image format supported by Mac OS X.
To email a picture from within Lyn, select some pictures and press the Mail toolbar item. Just make sure Mailplane is your default email client, the setting can be found in Mailplane > Preferences > Advanced.