Checking the existance of an image in a secure web page

I'm rather new to .net, and i never was a brilliant programmer in vb6, but i have a small problem (arnt they all).

I'm trying to write a code for a small utillity that nestles in the system tray, and can show 3 colors, and/or ring a bell. (wav, mp3). The colors are determind by the absence or presance of three individual images on a site that requires a login. The type of login is uncertain as of yet, but what would the procedure normaly be for the checking for the images?

I hope i can make sense of myself :-)