I have no idea of your level of skill, so pardon me for answering as if you're a beginner if you're not. That said, I use a free online collage maker called BeFunky. While it doesn't have the versatility of image editor software, it's easier to use and doesn't require a download or even registration, unless you'd want the upgraded version, but you don't need it for what I did.
The main things are:
1) Finding the pictures you want to use. Headshots are the best for this format. I have them saved in one file on my hard drive
2) Uploaded how every many images you want to the Collage Maker. They'll all appear on the left side as thumbnails.
3) Go to Formats, then Grids and select the style for how many images you want. In this case it was 16. There is also a 4, a 9, and so on.
4) Go to Customize and increase the Spacing to provide ample room for text. I'd suggest 80 or above. You can change the background color here too.
5) Drag your pics to the location you want them in the collage.
6) Figure out the size you want the full image to be. Mine was 2,500 px (pixels) by 2,500 px. Under Customize enter those numbers into the Width and Height. Do this before you add your text, or the text may shift.
7) Go to Text and add your writing. You can adjust the font size, colors, etc, and then place each line of text where you want it. It's kind of intuitive and will let you know when one line of text is lined up with another by showing you little lines and different colors.
8) When you're done save to computer, head over to Imgur to upload it, then you'll want to click on the image to get the full image address ending with .jpg. And then you can post the bad boy here. Voila.