First of all, you need to check "variants with individual price/sku/inventory". Then in the variants configuration screen, you have to follow the instructions precisely.
-------------------------------------
Enter the details of each variant in the box below (one on each line) in the following format:
SKU,Price,Ounces,Stock,Option1Value,Option2Value,Option3Value
Example:
GCS,9.00,32,120,Grey,Cotton,Small
BNL,9.50,34,100,Black,Nylon,Large
NOTE:
The number of Option Values you enter below in AT LEAST ONE LINE must be equal to the number Option Names you have entered above. SKU is the unique code using which you will identify the variant so you should use something which will help you recognize the product easily. SKU should only consist of numbers and alphabets.
-----------------------
So what you need to enter for your products is:
07-0130,$XXX.00,1,1,16 x 20 limited edition signed print
07-0125,$XXX.00,1,1,11 X 17 limited edition signed print
07-0115,$XXX.00,1,1,mini-prints signed
Since we don't know the stock or weight for your product, 1 is entered for both values. If you are not using our shipping or inventory control option, then it won't matter anyway.