Visit spark.adobe.com to begin. If you have an education account (you may have to speak to your IT friends), you will be able to save images without the watermark.
After logging in, select Graphic.
Pick a template or start from scratch.
Select your size. I chose a square for this example.
Choose a photo or upload your own. To be safe, and avoid copyright issue, I would find a royalty-free photo (or CC-0) before creating my awesome sticker.
Use the tools on the side to adjust your image, add text, change the colors, and other fun tools.
Click the download button, in the top right, when you are finished creating your masterpiece.
Sample