What you need:
1. O2Jam
2. Create a folder called "temp" in C:/. (Required or nothing will come out.)
3. Graphics editing software. I recommend Photoshop CS.
4. O2 Image Extractor(in the attachment)

Find a background. In this tutorial I'll use a COMBO as an example.
Put O2JamImageViewer.exe into O2Jam's image folder. (.../O2Jam/Image/)
Run O2JamImageViewer.exe and you'll see



If you look at the right side of the window, there are many filenames. These are the O2Jam's while-playing used images.
We will create a background to replace Bikini Beach, which looks really bad. On the midpoint of all those names is playing_BG10.ojs, which is the background image of Bikini Beach. This means that the Bikini Beach theme is the 10th theme in the game.

Now click on playing_BG10.ojs to highlight it.



Click 'Save to files' and this will pop out.



Now go to C:/temp, the folder you created and you will find file0.bmp Right-click on it, then select Properties. Select the Summary tab.


In the above window, we can see that this is a 596*558 pixels, 32-bit, 96*96 DPI BMP file. Now we know the background's picture's resolution settings.

Find the picture you wanted. In Photoshop, edit the canvas size to 596*558 and then add MYBG.PSD(included in the attachment) as a layer to turn it into something like this.


Now save it as a 32-bit BMP file and name it file0.bmp (Must be in this name). Put file0.bmp and bmp2ojt.exe in a folder and run bmp2ojt.exe.
(PS's Note: After some research, it seems that some OJT/OJS files had multiple frames. They are done by replacing 0 with another number. And then put the number of frames when asked in the program.)



Now press the ???OJT button. You will find a file called file.ojs in the directory where you placed bmp2ojt.bmp. Congratulations! We created the background!
Don't stop now, we need to replace the Bikini Beach in O2Jam for it to work. Rename file.ojs to playing_BG10.ojs, which is the filename we recorded in the beginning.

Now, run makopi.exe and click the Add button and find the playing_BG10.ojs you just renamed.



Press Pack. Name it playing1_300.opi. Open the IMAGE folder, and create a new folder called TEMP, then put playing1_300.opi in it.
Now, run O2Jam and it will patch the new file into it. Create a room and see your work.