Anna, no, not hard at all. There are probably better ways to do this than the way I do, but here are my steps anyhow: I resize the image to 400 pixels at the longer side, then sharpen and add a white border to create a 500x500 image. I then add a 1px black border to the right, resize and reborder the next image, and then tag that on at the side (again, I like working with borders, so I add another border of 500 pix in a diff color that I then overlay with the new image). And so on. I basically create one row at a time (saving as .tif while I work on it) and then compile those together, too. Then slap a big black border on, resize, sharpen one more time and voila :)
Now, people will read this and say "OMG, this can be done so much easier", but this is just the way I figured it out and it works for me ...
