I just tested out Seth's answer and it worked fine on my HTC EVO. I created the .nomedia
file using Notepadd++ and the default Notepad on Windows so both of those should create a .nomedia file just fine. Even though this worked for me I still suggest you make a backup of your files before trying this (I did when I tested) since from what I read this in an unreliable method for users to apply. Again from what I read it works fine if the media files are placed in the folder after the .nomedia file is placed in the folder but that if the files are there before the .nomedia file then it may delete the files. If Seth's answer does not work for you you may want to try creating an empty folder (with the same name as the one you want to put the .nomedia file in), place the .nomedia file in the folder and then add the photos (or whatever media you want) and see if that fixes the issue. I was going to try this if the first method failed but it hasn't deleted my media yet so we'll see what happens. If either of these methods are not helpful there are a few apps in the market that claim to hide your media. Again - BACK UP YOUR DATA FIRST - since I did read a comment that one user's photos were deleted.
Hope this helps
Edit: As Matt H pointed out I should have provided links. See the comments below for relevant links.