To have an image specifically created for this is impractical as it fits a rather small niche and to do as I describe below takes a matter of minutes, not hours. To reshape this limiting statement, I would reword it to simply say that one could have such an iso with Steam with very little effort. If you would like an iso with a lot more than just Steam added, you will have to wait a little while longer.
In the mean time and for your purposes simply installing Steam will suffice. Steam is after all available in the Ubuntu store and automatically installs its own dependencies. Since Ubuntu/Debian is where Steam is essentially made to work the best, it makes the most sense to use such a distro. You could grab any of the numerous already existing Debian based distros most of which incorporate the Ubuntu store and install Steam from there. It's really that easy. You could also download the .deb from steampowered.com and install it through the store that way.
There are other things that one using a Steam distro might also like to have such as controller support to which I cannot comment on existing support but I imagine most USB/USB dongle based controllers will work fine out of the box. Other than that, there might be various aesthetic things that one might like to see in a Steam distro such as icons/theme/wallpaper/login customizations. Thankfully that's all super easy to do on any Debian OS provided someone has already created something to your liking.
More specific to the question, you can make a persistent Ubuntu flash image that is bootable and simply install Steam on it. Plug it into any computer, boot it, install graphics drivers on host machine and load up Linux Steam.
If you have a 4-8+GB drive, you're all set. The larger than 4GB drive would help if you wanted to install games in the spare space for steam. If you had a 32GB drive, set the 4GB to linux persistent install and the rest is game space.
Steam can be set to run automatically at start like anything else once installed.
Alternatively, you could partition out ~30GB with GParted and install Ubuntu alongside your existing OS for testing purposes. There is the added benefit of the previous option being completely portable.
Even with the eventual arrival of SteamOS, it still holds true that it is extremely simple to set up nearly any Linux OS (primarily debian) with Steam though most would likely opt in for using the SteamOS instead (as would I - presumably - since it's not out yet) since it will have more features than simply distro + Steam.
SteamOS will be available for free as a Linux-based OS and will do everything you could need it to do including some claims of being able to play Windows/Mac games as well via a NVIDIA Shield-like streaming feature.