I want to use a personal picture as my splashscreen. How can I go about doing this.

Splash screen of what? Grub?

Im guessing it would be called a theme. It is at the log in screen. I have seen what my os and internet has offered, not what I want. Im interested in making my own theme.

And which login screen are you using? (kdm, gdm, …)

If you did a default installation with KDE, it should be kdm.
For that you can set the theme in “Configure Desktop”->“Login Screen”->“Theme”. You can even disable using a theme (on the “General” tab), and configure certain elements manually, like the background picture.

If that still doesn’t suit you and you really want to create your own theme, have a look into /usr/share/kde4/apps/kdm/themes/. Each theme has its own subfolder there. Copy one of the existing ones and change it accordingly (you should not modify an existing one because your changes would get lost when installing updates).
About creating themes, see here: http://docs.kde.org/stable/en/kde-workspace/kdm/kdm-themes.html