After thinking about how DMIs are really just PNGs with metadata comments, telling Dream Maker how to interpret them, I thought that it should be possible to give them thumbnails in the same way. I was surprised to find that it is very possible. With a simple tweak of the registry, I was able to give DMIs their own thumbnails in Windows Explorer, complete with a little Dream Maker icon in the lower right corner, kind of like PDF file previews! So this way, not only would they have intuitive thumbnails to show you the contents of the DMIs, but they would get cool looking DM indicators in the corner, to tell them apart from regular images.

I discovered that this feature is already possible, so I am suggesting that the ".dmi" key in the registry be updated with this in future BYOND installations (the very next one if possible). I believe that Adobe Reader uses something very similar for PDF file thumbnails, so I don't consider this to be a "hack".

Here are instructions on editing the Windows registry to get the DMI thumbnails:
Please note: I will not be held responsible for any damage caused by you editing your Windows registry!

Always make sure to backup your registry before making any changes. It contains information that is critical to running Windows!

Here is the easy, automated method:
Open notepad and paste this into it, keeping the quotes:

Go to file, save as, and under "Save as type:", change that to All Files (you must do this).

Give it a name, and after it include the extension ".reg". Save it somewhere you can find it and double click the registry file you made to apply the changes.

If all went well, you can enjoy your new thumbnails for DMIs in Windows Explorer!

Also, there may be an issue where the thumbnails don't show up unless you clean out the thumbnail cache. To do this, click start, type disk cleanup, and press Enter. Once it loads, scroll down until you see "Thumbnails". Just make sure that's checked and click OK to clean the cache. Now you should be able to see the thumbnails right away! Don't forget to show larger icons in Windows Explorer or you won't notice the thumbnails anyway.

For those who would like to do this manually (I don't recommend it unless you know what you are doing), here are the steps:

First, click start, type regedit, and press Enter. Expand HKEY_CLASSES_ROOT.

Then, scroll down until you see the key named ".dmi".

Next, right click the key, ".dmi", to create a new key under it called ShellEx (with no spaces).

Next, create another new key under "ShellEx" (the one you just made), and name it {BB2E617C-0920-11d1-9A0B-00C04FC2D6C1} You can copy and paste the bold text to be sure that it's correct, with CTRL-C and CTRL-V.

Finally, double click that long key you just made and look for the string in the right pane called "(Default)", which should be created automatically. Under Data, it should say "(value not set)". If that is not the case then you may have made a mistake and should apply the backup you made! Double click the "(Default)" string to edit it and type {3F30C968-480A-4C6C-862D-EFC0897BB84B} as the value data (or copy and paste it as before). Then, just click OK and the thumbnail icons should take effect immediately!
If not, then check the last paragraph of the easy method for a way to clean the thumbnail cache.

I have a feeling that a lot of developers here will like this tweak, so let me know if this is possible to implement as part of the next BYOND update/installation.
I find it really cool to be able to scan through the icons I make without needing to click on each one.
I tested this on Windows 7, so let us know if it works on XP, Vista, etc.