1, draw a picture you want to print on PCB by Photoshop or other Image designing tool. Pros and cons.Be careful about the Mirror effect of pattern on the back. Like this:

2,Transform the image you design into .bmp format and with a maximun of 256 colors.This is the rules of importing a BitMaP image into an EAGLE drawing:

3,After importing the image, the image will be on the layer200, turn the front side picture into layer 21, and turn the back side picture into layer 22.
4,Outline the border and make sure you have draw the outline on layer 20 for Dimension.Using the Arc and wire and circle drawing tool.
5,final step,overlap the two picture (pros and cons) together.and make your final PCB design board looks like this:

6. the last but not the least, don't forget to manufacture the PCB in two layers, not single layer! Even it only has one layer for routing. Because one layer will have no solder mask if you select single layer, which will make it much uglier.