It appears you are confusing the use of attributes and options. I believe what you want is 1 attribute(size) with 3 options (5x7, 8x10, and 12x18).

You'll want to setup this one attribute and 3 options, and then make sure that when you add this attribute to a product you also check off that you want to use these options. By default, an attribute will be added to a product without any options.

As far as prices goes, you'll be able to associate price adjustments with options. In this case you might want to set the product sell price as the price of a 5x7. Then in the 8x10 option and 12x18 option you can set the price adjustment that will properly add to your desired total price.