Because of the modular nature of dnn this not easily achieved. DNN does not need to store a record anywhere specifically of the places that a file is used. Instead each different type of module that exists in a DNN portal stores its own records of the files it uses - and the way that that information is stored can vary from module to module.
This is more complicated by the fact that HTML modules only store html markup - which might include links to files or might not - or might even include multiple links to the same file - and to complicate matters further could save those links in a couple of different ways depending on how the file was used or placed in the module.
an image can be linked directly - /dnn_platform/portals/0/Images/Banner2.jpg
or as a fileticket - /dnn_platform/LinkClick.aspx?fileticket=5IBm1dba-wA%253d&portalid=0
But either way its just HTML - other modules store just the FileID - 1234
or some might even store just a user path ../Images/Banner2.jpg
And exactly where the information is stored is widely varied - for example the image used by a user thumbnail is stored in a users profile - and accessed using its own path /profilepic.ashx?userId=64386&h=32 .
So to find all the places a file is used is bordering on impossible without some very complex sql queries.
However - if you do KNOW what the HTML looks like for an image you are trying to locate you could at least start by doing a sql query of the HtmlText table.
Select ModuleId from HtmlText where content like "%filename.jpg"
would get you the modules that it is used in.