linuxmemes

See, this is exactly my point in my other comment above. I could do this in about five seconds with Corel PhotoPaint.

1. Make a new document that's arbitrarily large.
2. Import both (or all 3, or all 10, or however many) images. (Images can be batch imported.)
3. Snap the first one to the top left corner.
4. Snap the others below it. Their corners and edges will click together if you have alignment guides enabled. 4a. Optionally resize any of the images by just typing in the value you need in pixels, in the toolbar when it's selected. If you need to know the size of any other image, just click it and it'll tell you. It's not even in a menu.
5. Crop tool (D) to knock the oversized canvas down to whatever size you need. Again, you can just type this in, in pixels, and it's not even buried in a menu.
6. Export, post, accumulate lulz.

Export to a flat format (.jpeg, .png, .gif, whatever) and your output will be flattened. You don't need to think about layers or merging or layers being bigger than the canvas or not. There is no, "Be careful not to XYZ." What you see in the preview is what the output will look like. Period. You can even apply your monitor's color calibration to it or the color profile of any other output device (printer, a different monitor, etc.) on the fly if you are a big enough nerd.

You can do this in an even simpler dumber way in CorelDRAW!

1. Import the images. Images can still be batch imported.
2. Arrange them however you want, snap them together, whatever.
3. Lasso them all and export.

That's... literally it. You don't have to crop, you don't have to trim, or layer, or anything. You can specify the dimensions of the output file in the export window before you hit save if you want it to be different than the original. Your arrangement doesn't even have to be rectangular and it will still work.