Hi Nik. Here are some ways you can hide pictures on your phone:
First option: Manual file management
If you don't have a stock file manager on your phone, you can get one from the Android Market, such as Astro File Manager, File Expert, or OI File Manager. Any of those will work. You can also mount your SD card on your computer and follow these steps from there (in fact, that way might be a lot faster).
Step 1: Open the file manager (or SD card) and add a new folder that starts with a period (.) followed by a name, like .mystuff
Step 2: Move your photos into this folder. They will no longer show in the default Gallery app nor any other picture viewer, just the file manager. You will need to remember where you put them, and you should note that they will not be protected by a password, as they would be if you'd used a photo-hiding app.
Second option: Apps that lock down and hide your pictures
There are a lot of options in this category. Two of the most popular apps are Vaulty and Gallery Lock Lite.