Why is this in Accessory Reviews? Regardless, the first thing you need to do is read this guide completely and download everything that it tells you to (
SSH on Windows - ModMyi - Wiki).
Next make sure that you have the new bubbles that you want on your desktop so that you can easily find them.
Once you installed OpenSSH and successfully SSHed into your device. Start clicking the folder at the top that is called ". ." to navigate up a level until you reach the level "/".
From there double click on the folder "System" followed by "Library". Once inside "Library", open up "PrivateFrameworks". Then once inside there, open up "ChatKit.framework".
In there, if you scroll down, you should see a bunch of png files (which are pictures). There are two you need to replace and they are called "Balloon_1.png" and "Balloon_2.png".
Now on your desktop make sure the two bubbles are named exactly the same as the two above. They are case sensitive so if everything is not exactly the same, they will not replace them.
The text bubble for your sent texts will be "Balloon_1.png" and the received ones are "Balloon_2.png". Click and drag the two new files into WinSCP. It will ask if you want to overwrite the old files. Click "Yes". Respring your device and you are done.
Edit: I attached the stock versions of the bubbles in case it didn't work out and you want to revert to the original ones.