UInv's DisplayItemList() function is only meant to display text, not images.
If you want to display your inventory as images, take a look at the "UInv_Sample_Code.html" file that comes with UInv. You can import that into Twine and then take a look at its code to see how you can create a table which will display your inventory. You can use the "Table Builder" in the UInv Help File to design how your table will look, and it will give you the CSS and HTML code needed to add that inventory table to your game.
If you don't want to use the inventory table method, then you could write your own code to display the inventory however you wish. For example, you can use:
[img["images/" + UInv.GetItemPropertyValue("inventory", "dagger", "image")]]
to display the dagger's image. If you want to make that work from within Twine as well, then you can add something like this to the top of your JavaScript section:
if (window.hasOwnProperty("storyFormat")) {
// Change this to the path where your HTML file is
// located if you want to run this from inside Twine.
setup.Path = "C:/Games/MyGame/"; // Running inside Twine application
} else {
setup.Path = ""; // Running in a browser
}
setup.SoundPath = setup.Path + "sounds/";
setup.ImagePath = setup.Path + "images/";
(Change "C:/Games/MyGame/" to the path to your game.)
Then, assuming your image is in the "images" subdirectory, you would do this:
[img[setup.ImagePath + UInv.GetItemPropertyValue("inventory", "dagger", "image")]]
and now it should display the dagger image correctly, both when playing it inside Twine and with the published HTML version.
In either this upcoming version or (more likely) the next one I plan to add the ability to add an easy to create shop interface, as the point of UInv is to try to make doing this stuff simple, even for "total noobs". ;-)
If you have any other questions on how to use UInv in Twine or any more general Twine/SugarCube questions, feel free to ask here any time, as I check this site pretty regularly.
Hope that helps! :-)