First you just need to save the picture of the text that you want.

To upload it:
1) Go to http://www.womrealms.com/
2) Sign in
3) Scroll down to where is says "Textures"
4) Then press "choose file" and select the picture that you saved earlier.

That's it, texture uploaded and available for use.

To use it:
5) Click edit next to your realm preview
6) Then scroll down to "Realm Worlds"
7) If you click "set theme", you can choose the texture pack that you just uploaded.
8) Save it all and it should be working next time you enter that realm.