I'm not sure which version of PhotoFiltre you have.
If it's the regular version open up a new image larger than your final image will be.
1) Copy and paste your first image into the new composition. When you do you will see dashed lines around the image. If you put your mouse in the center and left click you can move the image to wherever you want.
Move the image to the top left corner. Now move your mouse to the bottom right corner, Hold the "Shift Key", left click your mouse and resize the image to the size you want. You can see the actual dimensions along the bottom as you resize it.
2) When you have the image to the size you want, right click and choose validate.
3) Now follow that same procedure again, position the image below the 1st one, resize and validate.
4) When you have all the images you want in your composition, choose the rectangle Selection Tool, Draw a selection around what ypou want for your final composition and then go to : Image / Crop.
If you are using Studio just resize the layers as you add the images, merge them all together and then crop.