I like having all my snapshots with the same size. Now all the update packs have images in their original resolutions. So I've decided to create my own snapshots for the games I add to my collection.
As you know, MAME stores snapshots in their original resolution in files like 0000.png in a subfolder with the name of the game. I had to open the files, rename them, resize them and put them in my snap folder.
I've created a script to automate this. It uses ImageMagick, so you need this program installed in your computer (
http://www.imagemagick.org/script/download.php). Edit MAMEpaths.ini to define 3 paths. When you run the script, while in MAME, pressing F12 will save the snapshot, convert it to 222x167 (if the game is horizontal) or 167x222 (if it's vertical) and move it to the root of the defined snapshots directory, removing the folder created by MAME. Control + F12 will do the same but for title snapshots.
You can get the script from here:
http://www.alvarezeninternet.com/temp/MAMEsnap.zip. I hope you find it useful.