You can extend the colors of your logo by using the "Gradient Tool" and you can add Words with the "Text" Tool

Click on Gradient on the side tool bar. It will open up a tool bar on the top of the screen and will open up a color palette. If you click on the color option on the palette, and then move your cursor over on top of your logo color, you will get a "color dropper" which will automatically change the color for the gradient.

Once your color has been matched, you can drag your color across to the end of the canvas so as to fill the header.

You can add text on top of the header by clicking on the "Text" tool. You will have the option of changing font, colors, size and position.

Once your Header/Logo is complete, Save it on your computer, and then upload it to your media file or header file on your Word Press Theme.

I have found that this program works pretty well for resizing pictures for other Word Press requirements as well.

I hope this has helped! Happy Headers!