custom shape rectangle

Hello everyone,
I would like to create a word cloud with the shape of a rectangle (1920 x 495px), but between the forms there is not a rectangle and I saw that there is no longer an option custom shape.
I created a jpeg image in gimp, with the shape I need, but when I go to visualize tells me "Shape is empty!" how to do?
Thank you

There is a rectangle shape (looks like a square).
Click on it and the click on the green gear at the bottom right corner to adjust its ratio.