Photoshop usually does a pretty good job of creating efficient images, however, you can normally losslessly compress them further using Yahoo's smush.it tool or a command line tool.  Personally I use optipng which you can install via MacPorts.

You can also experiment with removing the @2x images entirely and just submitting with 2x resolution normal images. In most cases, the non-retina devices will downsample the larger images, but always worth testing on an actual device first.

Reduce image size without resolution change
I am just beginning to sell some of my images and would like to personalize them a little.  Many photographers sign and title their work on the white space just below the printed image. I am having metal prints made so there is no matting or white space, so I considered printing small (2"x3.5") stickers to place on the back of the images. What information do you think should be included on that sticker? Pretty much everything I've found online talks about a certificate of authenticity and limited editions. I do not want limited editions but would like to sign and number my larger prints. Does anyone have experience with this? Do I need to print a separate certificate? At this juncture my sticker will have my logo, image title, artist name (me), place for my signature and date, "edition #xx of an open edition". Some sites mention including the date the photograph was taken; format (i.e. 24"x36" photographic print on metal), though it seems redundant since it will be on the back of the print; same goes for description of artwork, as well as copyright as I am signing and dating it. I do plan on printing a second sticker with care instructions (i.e. hang indoors and out of direct sunlight etc.).  

Revision to clarify:
What information should be included? Do I need to put copyright information if I am signing it? Is it necessary to include the format of the print or the shot on date?  Should I also be providing a certificate of authenticity?

Thank you!
