every time i need to lay out several images next to each other I always put them in #inkscape
the infinite canvas and ability to resize any object at any time is more useful than you could ever imagine
(as well as the "clipping path" feature to cut off corners and fit more things in,
and "export bitmap" when you put a rectangle the size of your intended image at the bottom of your selection. i have taken a single raster image into inkscape & done this just because it's quicker than gimp)
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