User Agent or Feature detection in javascript for asp page

Hello:

I am back.  A couple of days ago I asked for help on an issue regarding the display of images in an asp page within cms teamsite on the uat4 server.  Well the cms developer says that the solution which involved <% if pageheader=true etc...%> with an include asp file will require a component change and will take several days.

The business wanted this asap. She says using javascript would be faster.  She is the one who failed to provide information that FISERV is passing some of the parameters.  

So now I am back to square one.  The issue they want fixed is, if the device is a older windows phone (using IE 8) that the low res image would show up.  The way this is being done is by passing a parameter esign.asp?pageheader=true and the logo shows up.

Now the lead on my team wants to have high res images show up on unsupported tablets

I am only getting vague help from the offshore team that built this code and they are saying this issue is "out of scope".

It would seem that the best approach here would be to detect a device's width, any device at all, and go for the low res image, and if it is landscape, tablet or desktop put in the high res image.

If I do this, I must do it in a way that will not impair their byzantine set up.

So, kind people, can you point me to some code I can use that will cover all devices and point to either the low res or high res image in these pages?

http://uat4-m.usbank.com/mobile-banking/edocs/disclosures/esign.asp
http://uat4-m.usbank.com/mobile-banking/edocs/dp/terms_conditions.asp

Thanks as always, for your help

Regards,
seeker