merging two images with triangle crop

In Eagle, I've created a circuit that consists of schematic and PCB layout. From Schematic and PCB Layout I've created screenshots in .jpg format. Now, how do I merge these two screenshots with triangle crop just like in example below (left image, sch&pcb merged): following example . I am using KUbuntu 15.04 with gimp, pinta and inkscape image manipulation applications. Oh, I forgot, the screenshots are neither same ratio neither same size.

Solution

If the images are in the same ratio, you can use GIMP:

open both images in gimp
select smallest image and copy it to the clipboard
paste clipboard content as a layer into the largest image
transform/scale layer (shift + t), find a best mapping with the background image (you can hide a layer with click on an "eye" in layers window, use lock scaling if appropriate for your image ratio)
in an overlapped layer select a triangle, and press delete button