yes, the [DOWNLOAD] token injects a linkButton control. You can set the text used for the download button in the resource file via the DownloadButton.Text resource key.

So, now here's the magic :)   since the text will just be rendered as text and it supports html tags, you can actually set the text for your download buttons to an img tag, thereby rendering an image for all your download buttons, but still retaining all the functionality of a tooltip, click counting and triggering the download.

there are a couple of things to pay particular attention to..

1. the path to the file needs to be absolute or if it's relative you need to be aware of the current directory
2. in order to enter the html tags in the resource file, I believe you need to wrap the img tag in a CDATA construct

Here's an example:
In the /DesktopModules/Repository/App_LocalResources folder, edit the Repository.ascx.resx file and search for the DownloadButton.Text tag, change it like this...

Now, all of your download buttons will be rendered with the download.gif file (in the above example, of course you can use any image you want).  Notice the relative path, it goes 3 levels back and will look for the image in the ~/images folder.

If you want to make the path absolute, you can do so, you'll have to play with the path to get it right for your particular setup and where you store your image.

it's not so easy ..

In addition to adding code to support a new token, you'll have to add a new column to the repository database to store the new image, or if you're okay with a single image for all repository items, then you'll have to modify the module settings code to save/restore a new setting indicating the image to use.

I plan to do a little video blog on how to add a new token very soon ( hopefully sometime this weekend ) which will cover most of the work. The code to support a new module settings is pretty easy as well. So, after I publish the token video, let me know if you're still itchin to make the change and I'll help you out with some sample code for the module settings.