Share with Google Sites
Create a new Google Site for your stickers.
Click the Pages tab on the right and click the plus icon to add a page. Enter a name. Click Advanced and give the page a path that students will not guess. Click Done.
In the right side panel, hover over the name of the page you just created. Click the three dots and click Hide from navigation.
Click the Insert tab on the right. Click Images and choose Upload.
Click the Publish Button. If this is the first time you are publishing the site, you'll need to give it a URL.
After it's published, click the arrow next to the Publish button and click View published site. (Here's an example of a published page with a sticker.)
Copy the URL for the page. Give this link to students who have earned the sticker.
When a student opens the link, they will right click (or two-finger click) the sticker and copy. Then they will open their sticker sheet and paste.
Share with Google Drive
Save the PNG to Google Drive.
Right-click (or two-finger click) the file in Google Drive and select Get Link. Change Restricted to Anyone with the link. Click Copy link.
Give this link to students who have earned the sticker. (Here's an example of a shared link to a sticker in Google Drive.)
When a student opens the link, they will need to click the Download icon at the top right of the tab. After downloading, it can be inserted into Google Slides.